Unlike the precedent for moving crypto without the keys we saw with the Ethereum DAO hack case, Bitcoin(BTC) would not need a re-org.
It would be safe and more professional to re-assign the assets and maintain the current blockchain as is.
It important to note a re-assignment is not the same as a re-org.
Reassignment is an new transaction type, adopted by patching the miner code. This new type would take the form of a new OP_CODE which spends the lost asset to a new address owned by the legal owner
And just like with an accountants book, the OP_CODE notes this change in the transaction by recording a correcting journal entry.
The original and illegal transaction remains immutable preserving the blockchain integrity.
No re-org is needed.