3926 Mutatis Mutandis by Nibor

Nibor came up with an original way of cluing inspired by the title, which means -with necessary changes. The clue gave an answer that had to have one letter changed and then jumbled to give the grid entry. Definitions of grid entries had been inserted into clues, which were all otherwise normal. The hints in the preamble gave the letters to be removed in clue order and then the letters to be added. This made a difficult task much easier and allowed for a neat sense of satisfaction at the end.

PREAMBLE DEFINITIONS - REMOVED LETTERS
expressed surprise OOHED
haughtily SNOOTILY
very deep HADAL
marsh FEN
edge CURB
testing ASSAY
bird NANDOO
(Sassenach) LOWLAND

PREAMBLE DEFINITIONS - ADDED LETTERS
to inspire ACCORAGE
restaurant CREPERIE
moved plants REPOTTED
birds VEERIES
computer CLIENT
beaters CANERS

The answers to clues are followed by the grid entry and definition in brackets.

ACROSS
1 Indian farmer, bohemian perhaps, is a hoot, we hear (4)
RYOT (ARTY bohemian perhaps) sounds like RIOT
4 Cry of joy - low in calories, this mineral extract (6)
IOLITE (ELICIT extract) io + lite
11 An event he's arranged with transient divine intervention? (10)
HEAVEN-SENT (EVANESCENT transient) anag. an event he's
12 We assert without hesitation: "edited", rarely acknowledged, means "fabricated" (6)
WEAVED (AVOWED acknowledged) We + av(er) + ed. (edited) Weaved is a rare past tense)
13 Fashionable gods initially could take charge of gaseous element (6)
INDICT (NITRIC of gaseous element) In (fashionable) + di + CT (initial letters of take charge)
14 To harm swallows without feet for little markets? (8)
MARTLETS (MALTREAT to harm) Swallows without feet in heraldry might also be mart-lets or little markets!
15 Partially mishandles phoney extra coin (4)
SHAN (GASH extra) hidden
16 Wiseacres smuggle a large number to Sweden (4)
OWLS (SLEW a large number) OWL (to smuggle (esp wool or sheep from England to France) + S (Sweden)
17 Endlessly arrange with dexterity a right-wing spin machine (8)
FILATORY (CRAFTILY with dexterity) Fil(e) + a Tory
20 Rabbit backs return to main channel of communication - Scottish river (6)
YATTER (ARTERY main channel of communication) rabbit in the sense of talk RET + TAY reversed.
22 Run away from dear marijuana by the sound if it (4)
LIEF (FLEE run away from) sounds like LEAF (marijuana (slang))
23 Asthma aid bust her nail - it delays the clot (7)
INHALER (HEPARIN it delays the clot) anag. her nail
26 Diminutive manly chap with Yankee dame (4)
ANDY (EDNA dame ref. Dame Edna Everage) Andrew (from the Greek means manly) AND (with ) + Y (Yankee)
28 Audibly gets money in short supply from stores... (6)
CACHES (SCARCE in short supply) sounds like CASHES
29 ... and maybe schoolmasterly Astaire used to get it from diamonds at one time (8)
TAPDANCE (PEDANTIC schoolmasterly) TAP to get (money) from + D (diamonds) + ANCE= once
30 Formerly fix limited parking to one side of field (4)
PEND (PEEN fix) P (parking) + END (see Chambers) Pend (old form of penned)
32 Pulled along unfinished bundle, half dead? You'd need wellies to do this (4)
WADE (DREW pulled along) WA(D) (bundle) + DE(AD)
34 Part of lock-gate frame (it's pretty aromatic) closes promptly with difficulty
CLAP-SILL (ALLSPICE it's pretty spicy) CLAPS closes promptly + ILL (difficulty)
36 Flap of skin in which you'd get Alsace and Riesling to start with a restaurant? (6)
CARAFE (EAR-CAP flap of skin) A(lsace) + R(eilsing) in CAFÉ (&lit.)
37 Equiangular object, elegantly simple, goes in crookedly (6)
SOIGNÉ (ISOGON equiangular object) anag. goes in
38 Experimental driller let WC drain - awfully well covered up (10)
WINTER-CLAD (WILDCATTER experimental driller) anag. let WC drain
39 Eerie shout outside - go back inside to print again (6)
CREEPY (RETYPE to print again) CRY (shout) round PEE (rev.) Gregson smiled at this meaning of go - to urinate!
40 Hindu books put gold in very dead region of Switzerland (4}
VAUD (VEDA Hindu books) AU (gold) in V (very) and D (dead)

DAWN
1 Will's gaoler, a South American creature found in Panama? Rarely (4)
MARA (ADAM Will (Shakespeare)'s gaoler) Hidden
2 CD competition collection, say, has balance, with Rameau and Yradier for starters (7)
LIBRARY (RIVALRY competition) Libra (balance) + initials R & Y
3 Indian vessel reluctant to take one pin for oars on board (5)
LOTAH (THOLE one pin for oars on board) A (one) in Loth (reluctant)
4 Senior Guides, always fresh, lose one but get even, that's what they do (9)
REVENGERS (EVERGREEN always fresh) Rangers (minus AN replaced by EVEN)
5 See preamble (13) The unclued entry at 5 that does not apply to the theme of this puzzle. INVARIABILITY
6 Mythical creature, a headbanger, gives teachers a set of pistols, say (7)
NUTCASE (CENTAUR mythical creature) NUT (National Union of Teachers) Case - a set, eg of pistols in Chambers
7 Egyptian goddess? It's a load of monkeys (4)
SAIS (ISIS Egyptian goddess) SA (it - sex appeal) is
8 One to cause an accident in an uninteresting way when attention wanders? (8)
DREARILY (DERAILER One to cause an accident) EAR (attention) in DRILY &lit.
9 To isolate under control when wearing linen cloth (6, two words)
INISLE (IN LINE) in LISLE cotton yarn
10 Oriental fish swallows drug, an irritating sensation (4)
THAI (ITCH an irritating sensation) H (heroin) in Tai (a Japanese sea bream)
16 Pinch orchid tubers being returned before a meal (5)
SNEAP (SALEP orchid tubers) ENS (being) rev. + AP (ante prandium (Latin), before a meal
18 Cafe - dish of rabbit perhaps - dress adjusted for strolls in late evening? (9)
FRESCADES (FRICASSEE dish of rabbit perhaps) anag. café dress
19 Arab state wealth is my only yen (5)
MONEY (YEMEN Arab state) M' + ONE + Y (yen)
21 Older yet potentially blocking things out gently (8)
DELETORY (TENDERLY gently) anag. older yet
24 Apparently fluttering partner's initially to introduce binding agreements (7)
APPALTI (PITAPAT fluttering) AP (apparently) + PAL + initials T + I
25 Of old extremely insipid, including film of small freezing leopard (7)
OCELOID (ICE-COLD freezing) O' (of) O (old) +CEL (film) in ID (extremes of InsipiD)
27 Casement of Squeers's painter, needed when on watch? (6)
WINDER (DERAIN painter) Gregson took this to be a Cockney version of window. Squeers runs Dotheboys Hall school in Nicholas Nickleby.
31 Ambassador's no right to be a first born lord (5)
LIEGE (EIGNE first born) LIEGE(R) an ambassador
32 Moist river valley's reportedly an object useful in warfare (4)
WADY (DEWY moist) sounds like WADDY an Australian Aboriginal wooden club used in warfare
33 Spots a stock bollard -- none is missing (4)
ACNE (RACE stock) A cone minus O
35 Head of college entertainment association would give birth (4)
DEAN (ENSA entertainment association) D (would - as in I'd) + EAN to give birth