early migration could be done right, if it's optional at the script level, at the time of spending.
but we would need to change how keys are generated and handled as standard, etc...
the chain that does not confiscate non-upgrading coins will go to $0, the chain that confiscates will not be #bitcoin (and probably also go to $0 just slower).
that's why i'm saying, just pray that quantum is fake and gay!