What change are you trying to make? Why don't you call to try and make the change without losing the seats?
No-one can tell you what the chance of the seat returning to award inventory is.
I had a one way JNB-LON (for 2 people)
Wanted to add an outbound, and a 2-4-1. BA said could only be done by cancel and rebook.
Anyway, to answer my own question. I went ahead and did it. The JNB-LON did NOT return to inventory, so I've lost the seats. Agent said there was still plenty of J class inventory, but it was a peak time - so presumably that was a factor in them not returning.