Old clients will refuse to relay sendmany transactions
Using this way anyone with new client can doublespend money - send once to client with new version, then send second time to the client with old version.
If someone confirm the second transaction (which is theoretically possible, while not everybody miner update their software), the money will be doublespent.
If I can setup old and new client in different virtual machines - will I be able to double my money? and repeat that again in 10 minutes?