Except in this case individual users will also grow. You'd have to do something like grant them their current usage plus some buffer (without letting them know ahead of time maybe, since otherwise this is gameable).
Why do you need a buffer? If you’re grandfathering in old and limiting new users more, what is gameable? Besides a small portion of people registering accounts in the short time between announcement and implementation?
They could have grandfathered the existing accounts into the 2000 minutes. Then the segment would no longer be growing and stay small, no?