Most of the existing versioning proposals involve starting the contract with a currently invalid bytecode and interpreting what follows as some sort of version name or number. If we don’t want to sort through them all, reopen their discussions, and get consensus on a general scheme, then we can solve the problem just for this proposal.
We can insist in EIP-615 that the implicit main routine that begins each contract must instead start with an explicit, BEGINSUB 0,0. That marks post-615 code and makes it invalid to pre-615 VMs.
In Phase One only post-615 code must be valid. In an optional Phase Two we stop allowing pre-615 contracts at all, except as created by pre-615 contracts.