Interchangeability of parts, air frames made with new tooling have no guarantee that parts for one series of aircraft built with the old tooling will fit to air frames built with the new tooling and vice versa.
Think cheap aftermarket Chinese replacement fenders for your bent-up car. They don't quite fit right. Bad enough for your soccer mom mobile, potentially fatal under the high stresses of combat.
The A-10 was designed with the intent of being able to take undamaged parts from unflyable birds and 'Frankenstein' them together in the field and get them back in combat. Keeping track of which salvaged wig can fit which series would be a logistical nightmare.
So why can't we re use the old tooling?
It was ordered destroyed at the end of the first run of the contract.
Top Airforce Brass really hates the A-10...
The A-10 will take tremendous damage and bring the pilot home. No question. But the other problem is a stat that doesn’t get reported — how many of those 700 original airframes have already been written off due to battle damage? I don’t know, but I suspect that the cumulative number is between 20 & 40%. These planes have been in heavy use since the first Gulf War.