brewerdave,
That's fine if aircraft changes were a regular thing, but they aren't. The vast majority of flights operate as scheduled. I know aircraft changes shouldn't occur, but it's inevitable some changes will occur throughout the year for the reasons mentioned above. Aircraft changes are as much a nightmare for the airline as they are for passengers! You can't please everyone all of the time. If only 31 premium seats were sold per flight out of 63 in case an aircraft is changed, it would be a struggle to sell them all at last minute, especially because seating is pre-allocated to speed-up despatch and check-in. Imagine the size of the queue if up to 32 passengers started to change their bookings, sent to another desk to pay the extra etc. It wouldn't work.
The other issue is aircraft capacity, there's a big difference. There are 283 (31Y+ / 252Y) seats on the ex-TOM 76W's, but only 258 on the ex-FCA 76W's (63Y+ / 195Y). It would mean you'd either overbook on one aircraft, or underbook on the other. With the different seating capacities, as you can imagine, you don't want to be in a situation where you plan for a 283 seat aircraft and have to bring in a 258. It's not too bad the other way around, at least everyone gets a seat, just unfortunatley not the ones they may have booked. Airlines need to be sending out full aircraft, hence they are planned to use a particular one well in advance and don't want to be changing them unless it's
absolutely necassary. Things aren't always as simple as they seem.
The real answer is aircraft commonality, ie have them all in the ex-FCA or ex-TOM then you can swap and change as much as you want. But again, it's not that simple as some routes demand more premium seats, some more economy seats, hence the mixed configurations.
Darren