You're beginning to lose me here...
Aircraft A crashes and gets rebuilt using some original parts (the unbroken ones) and lots of new parts (replacing the broken ones). It is still aircraft A.
It can retain its original registration or it can transfer to a new registration if you like - but the original registration has been allocated to aircraft A and can thus never be used on any other aircraft - but could be used again on aircraft A in the future should the owner want to revert to it.
There is no other aircraft involved.