mechanically and operationally the 321's are interoperable with 314,318,320 & 322, so there will be no problems operating, as for depot maintenance they have experience with the brush equipment on their sister fleets, so no problem.
remember that 319 are GEC equipped, with same motoring as 317,318 and 325, but are able to work with 321 without issue
320-322 are essentially the same fleet, only minor differences that will affect drivers are their max speeds - 320's I think are 90mph now??
Making a 321 a 3 car would require compressor relocation and some wiring & pipe moves & changes, once one is done, should not take too long per unit to do..
be a shame to see the old girls go, they have been the longest serving fleet on the Euston - BNS regional lines...they have plenty of life in them, and as said will easily have a half century or more life in them..