You could for example bucket them based on the last digit of their IP address. So, everyone whose IP address ends in 0, 1, 2, 3 or 4 gets the new version, ever

You could for example bucket them based on the last digit of their IP address. So, everyone whose IP address ends in 0, 1, 2, 3 or 4 gets the new version, everyone else gets the old version. You don't need to store the IP address for that, you just need a rule that decides which version to serve as the user requests it.