The ISPM 15 view
Repair must use only wood treated to the standard, or processed wood. Where treated wood is used, each added component must carry its own mark. Alternatively the national authority may allow the whole unit to be re-treated, in which case old marks are obliterated and a new mark applied. ISPM 15 notes that multiple marks can make the origin hard to judge.
The EPAL view
EPAL licenses repairers separately from producers. A licensed repair uses dimensionally correct, treated parts and adds the repair marking nail. EPAL's poster treats a repair not to specification as not fit for use.
What it means when you sell
A licensed repaired EPAL pallet can be class A, B or C. An unlicensed repair is a not-fit-for-use EPAL pallet, whatever it looks like. Sell it as non-EPAL wood or recycle it.