Thanks chaps, sorry it's taken a while replying.
Engine changes during a major overhaul aren't a surprise, and I'd have thought were probably standard procedure. Engine changes in the event of a catastrophic failure aren't a surprise either. It's the extent to which engine changes went on in the sort of circumstances Richard mentions that I was interested in, I suppose.
I could see that if parts weren't available, or where an engine change would be quicker than a repair (particularly on exercise or where the vehicle was needed quickly) then engines would be swapped. I assume it's likely that in those circumstances the engine change wouldn't be recorded, or at least the repair wouldn't be entered on the military chassis plate.
The reason I ask is because I'm still niggling at this business about engine colours with particular reference to Austin K9s. I'd thought it was possible that Austin were painting K9 engines in eau-de-nil ex-factory as virtually every K9 engine I've seen was eau-de-nil, and I've seen a fair few, and often in K9s where there was no sign or record of an engine change or significant repair. The same seems to be true of many other vehicles.
However, I've just found that the engine in at least one of my K9s, which has no repair on record, isn't the original one for the vehicle (or at least, isn't the one the MoD have on record as originally fitted). That suggests that perhaps engines were being changed almost as routine during the 50s and 60s at least.