...When someone spends on Bitcoin-A, that person will also have to replicate/translate that transaction on Bitcoin-B...
...Bitcoin A's history goes to the trashbin, but the users can, say, rollback a few days and switch into Bitcoin B...
If the transactions in Bitcoin-A are exactly replicated in Bitcoin-B, then they are subject to the same vulnerability, and therefor Bitcoin-B would go to the trashbin right along with Bitcoin-A.
If the transactions in Bitcoin-B are
required to NOT have outputs that use ECDSA, then what's the purpose of keeping Bitcoin-A around at all? A user can't create a Bitcoin-A transaction without ALSO creating a non-ECDSA transaction, so why allow the ECDSA transactions at all?