I am not sure if I understood your comment. Miners cannot apply old semantics when the transaction has an old version field, because that field can be faked by the clients to sabotage the change. E.g., suppose that the change imposed a mininum output amount of 0.0001 BTC as a way to reduce spam attacks on the UTXO database. An attacker could frustrate that measure by issuing transactions with the pre-fork version tag. Does that answer your comment?
I don't buy the argument about "frustrating that measure". It is very easy to verify that the "old style" transactions use only "old coins", the coins that were confirmed no later than the effective time of the new transaction format.
Theoretically someone could try to launch the attack using only the "old coins", pretending to have a pre-signed transaction with some rather large n-lock-time. I think that type of attack would be self-extinguishing, it could be launched only once for each "old" UTxO entry.