Pairing Off solution
by Kegler

As indicated by 23 Across, DOUBLE SWITCH, every answer with a doubled letter exchanges it for a different pair.

Pairing Off solution grid

Across
1O + DDS {DD->FF}
5F + LOGGED {GG->SS}
11VEAL (hidden)
12ODE ("OWED" hom)
13BOULE (1st letters)
15S(NOW) + M + AN
16RUBy + BING {BB->NN}
18B(R)EECH {EE->OO}
19W + O + ODS {OO->EE}
21BISHOPRIC (I CHOP RIBS anag)
23DOUBLE SWITCH
25D + RAIN
26CAN + DID
28ITSELF (STIFLE anag)
30P(IT + M)EN
33TI(PTO)E (POT anag)
35G + NAT (rev)
36FLI(V + V)ERS {VV->PP}
38H(OUR)I
39EMBALMS (BLAME MS anag)
40EG + G + ON {GG->XX}
41DAP + PLED (PAD rev) {PP->ZZ}
Down
1cOVER(BILL)ING {LL->DD}
2FEMUR (hidden)
3FA(WNO)N (NOW anag)
4CODICIL (rev hidden)
6LEN + G + THe
7SO + W
8SUPPERTIME (PRESUME TIP anag) {PP->MM}
9E(LA)ND
10DENTS (odd letters)
13BOR + ROW (ROB rev) {RR->WW}
14KNOTTIEST (hidden) {TT->BB}
17TE(P)ID (DIET rev)
20FOR + TKNO + X (KNOT anag)
22CHIN + A
24SAP (rev)
27DEAR ME (REAMED anag)
29F(I)LED
31TOP + AZ
32FAUX ("FOE" hom)
34PI(M)A
36FIN (2 defs)
37E + B + B {BB->LL}