With a 1wp we cannot expect all coins to move to the new chain -- some wallets are lost for example, so when do you stop supporting old UTXOs? With a spinoff or hard fork it would be possible to inherit the old UTXO set, but at the cost of not being able to deprecate handling the old chain data structures. A sync from genesis requires understanding the old chain up to a certain height, then switching to code that handles the new chain. Alternatively you can dump the UTXO set and import it at the changeover height, but do you still handle old style transactions at that point? Do you flag day ex