A welcome return to eXternal, not seen since Twixmas.
Preamble: Wordplay in each across clue generates a letter not required for the answer; in clue order, these give information about an event. Where there are two down entries in a column, a single letter must be removed from each answer to form the entry, wordplay leading to the mutilated entry. In column order, letters removed from upper entries give one thematic name and those from lower entries give another. Finally, solvers must change two symmetrically placed entries to show a change of title (3 words) after the event. All final entries are real words or phrases.
I went through the across clues with moderate success, but was halted briefly by the entries for the first few down answers – not real words. But all felt OK when I quickly remembered “All final entries are real words or phrases”, and doubtless matters would be resolved by the changes to be made in the endgame. 
The down entries in the upper right side were real words though, as were those in the lower left; however, some entries in the lower right weren’t, but that’s where the symmetrical placement of the changes would bite.
The different manipulations for the across and down clues/answers/entries wasn’t really a problem when going through first the across clues and then the down ones, but when it came to tackling the remaining, say, 25% which involved dotting back and forth between across and down I found it required quite a lot of concentration. 
What would emerge as GRACE KELLY and RAINIER III had been clear for some time, but I was a bit slower to make complete sense of the information coming from the across clues – just ever so slightly too fragmented. But before too long I had CURRENT DAY MONTH AND I?NUMBER, and it was obvious that the ? had to be Q, but what to make of IQ NUMBER? I’d already clocked that the pair had got married on 18 April 1956 and that the publication date of the puzzle was 50 years on … and then the head-slap – 1956 is the puzzle number (and IQ = Inquisitor).
Current entries at 2d, 3d, 4d, 5d & 7d up top, and 36d, 37d & 34d down below weren’t real words so the entries to change had to fix that which meant we were targeting 13a and the symmetric 43a. A quick check and sure enough PRINCESS OF MONACO was the change of title that did the trick.
Thanks, eXternal. Not that difficult a puzzle, but well-engineered. It probably wouldn’t make an anthology unless all the puzzles indicated the full date they were published and the publication they appeared in.
| No. | Clue | Answer | X | Wordplay |
| Across | ||||
| 1 | Spicy mixture in grandma’s calamari (6) | MASALA | C | (grand)MAS [C]ALA(mari) |
| 6 | Literature’s one striking eastern truism translated (6) | SMITER | U | [E(aster) TR[U]ISM]* |
| 11 | Every so often, carer can’t guard beastie (6) | CRATUR | R | C(a)R(e)[R](c)A(n)T(g)U(a)R(d) |
| 12 | In Durban, bits of bramble from sides of ruin blocking entrances (6) | DOORNS | R | R(ui)N in DOO[R]S (entrances) |
| 13 | Proclaimed dire need to protect snow leopard (8) | ENOUNCED | E | [NEED]* around OUNC[E] (snow leopard) |
| 14 | Tons entering square for evening (4) | NITE | N | T(ons) in NI[N]E (square) |
| 17 | Highlander’s garment bearing a cabbage (4) | KAIL | T | KIL[T] (highlander’s garment) around A |
| 19 | The German knocked back vermouth and wine (4) | ASTI | D | [D]AS IT< (vermouth) |
| 20 | Scots seal is turning round island within loch (6) | SILKIE | A | IS< I(sland) in L[A]KE (loch) |
| 21 | That woman brought in by Curie, coy nurse (7) | CHERISH | Y | HER (that woman) in CI (Curie) SH[Y] (coy) |
| 23 | Ancient portico protected by six mystics (4) | XYST | M | (si)X [M]YST(ics) |
| 24 | Jesting to oust joint ruler (4) | KING | O | J[O]KING (jesting) ¬ J(oint) |
| 27 | Costard’s errant utterance mangled German name (4) | EGMA | N | [G(erman) [N]AME]* |
| 29 | Part of speech set forth in middle of dinner (4) | NOUN | T | OU[T] (set forth) in (di)NN(er) |
| 30 | Bones of American steed start to ache (4) | OSSA | H | [H]OSS (horse, US) A(che) |
| 31 | Love to photograph old mate (4) | OPPO | A | O (love) P[A]P (photograph) O(ld) |
| 33 | Cunning plans Lynne’s sabotaged to some extent (7) | SLYNESS | N | (plan)S LYN[N]ES S(abotaged) |
| 35 | Father and another engrossed in Royal Engineers display (6) | PARADE | D | PA (father) [D]AD (father) in RE (Royal Engineers) |
| 38 | Side petals of each lilac stripped all round (4) | ALAE | I | EA(ch) (l)[I]LA(c) all< |
| 40 | Returned extremely queer hollow turnip, say (4) | ROOT | Q | TOO (extremely) [Q](uee)R all< |
| 42 | Vanish shortly after ordering travel document (4) | VISA | N | [VA[N]IS(h)]* |
| 43 | Unsafe in use, remedy (8) | INSECURE | U | IN [U]SE CURE (remedy) |
| 44 | Male attendants worked as lackeys boundlessly and no longer weaken (6) | ENERVE | M | [M]EN (s)ERVE(d) (worked as lackeys) |
| 45 | Humorous ridicule grasping large feature of stag (6) | ANTLER | B | [B]ANTER (humorous ridicule) around L(arge) |
| 46 | Assented to get on grass (6) | AGREED | E | AG[E] (get on) REED (grass) |
| 47 | Most recent remake of Western (6) | NEWEST | R | [WESTE[R]N]* |
| Down | ||||
| 2 | Youths in race upcoming not all trim (6) | [G]ARNISH | G | (yout)HS IN RA(ce) rev. |
| 3 | Perform when lifting Indian stringed instrument (4) | SA[R]OD | R | DO (perform) AS (when) all< |
| 4 | Awkward footman losing head (5) | [C]LUNKY | C | (f)LUNKY (footman) |
| 5 | Regularly displaying fabric, handy place for shoppers (5) | ARCAD[E] | E | (f)A(b)R(i)C(h)A(n)D(y) |
| 7 | Restless amidst limitlessly long sittings in the Capitol (8) | MOD[E]LINGS | E | [(a)MIDS(t) LONG]* |
| 8 | Courageous folk close to stopping crisis at midpoint (4) | [L]IONS | L | ON (close to) in (cr)IS(is) |
| 9 | Running irregularly Tuesday, gutted carrying sprain (6) | TRICK[L]Y | L | T(uesda)Y around RICK (sprain) |
| 10 | Would-be political influencers sit with stern rogue (8) | ENTR[Y]ISTS | Y | [SIT STERN]* |
| 15 | Second Hitchcock film is piece of tack (8) | BACK-ROPE | BACK (second) ROPE (Hitchcock film) | |
| 16 | Heroic woman to run after Queen Victoria (5) | VIR[A]GO | A | GO (run) after VIR (Victoria, Empress and Queen) |
| 18 | Transfer system later redeveloped and regulated (8) | LETRASET | [LATER]* SET (regulated) | |
| 20 | Square part of loaf is bread for Israeli (5) | SHE[K]EL | K | S(quare) HEEL (part of loaf) |
| 22 | Mac’s comely offspring with vacuous spouse (5) | SONS[I]E | I | SON (offspring) S(pous)E |
| 25 | One friend involved in stink holding legal consultation (8) | IMPA[R]LING | R | I (one) PAL (friend) in MING (stink) |
| 26 | Div confused with route surpassed in motoring (8) | OUTDRIVE[N] | N | [DIV ROUTE]* |
| 28 | Flash new aluminium mullion (5) | MON[I]AL | I | MO (flash) N(ew) AL(uminium) |
| 32 | Positive upright commendatory person (6) | PR[A]ISER | A | P(ositive) RISER (upright) |
| 34 | Long fish are about third of haul aboard ship (6) | SAUR[I]ES | I | ARE around (ha)U(l) in SS (ship) |
| 36 | Mineral found in guano’s analysed (5) | NOS[E]AN | E | (gua)NOS AN(alysed) |
| 37 | Uptight promoting special novelist (5) | STE[R]NE | R | TENSE (uptight) with S(pecial) earlier |
| 39 | In the morning turning on mayor of Lille (4) | MA[I]RE | I | AM< (in the morning) RE (on) |
| 41 | Deceit putting base below bottomless abyss (4) | GU[I]LE | I | E (base) after GUL(f) (abyss) |
An excellent, well-conceived, well-constructed and well-clued Inquisitor. I found the Down clues generally a tad harder to solve than the Acrosses. I came to a stop with just 38 ALAE to solve, leaving a gap in the collected ‘extra’ letters with ‘Q’ to follow it – a ‘problem’ very similar to that noted by the blogger! That pair of letters surely had to be HQ or IQ, and it took me also longer than it should have to see the meaning of ‘IQ NUMBER’. Given GRACE KELLY and RAINIER III (arising from the Down clues), PRINCESS OF MONACO was an obvious guess at the new ‘title’, and I decided simply to try the three pairs of symmetrically placed eight-letter words in turn to see which of them would accommodate the change, incidentally forgetting that there were a few non-words to resolve in the grid. It turned out to be the first such pair I tried (ENOUNCED/INSECURE) – a very satisfying outcome.
Thanks to both eXternal and HolyGhost.