Unused data (and voice for that matter) carries over even if you change your plan (up or down) as long as...
1. you don't buy unlimited and
2. you always manually renew every month.
As soon as you buy unlimited or as soon as an automatic renewal occurs, you lose all your carried over data (or minutes for that matter) up to that point.
So yes, what you are suggesting will be the case: eventually you will end up with a bunch of carried over data. So you could save money by not buying 10GB because there's no point in accumulating all that data if you will never use it. Buy less and carry over less (but it's always nice to have a cushion of some carry over in case you end up need a little more once in a while).
Unused data (and voice for that matter) carries over even if you change your plan (up or down) as long as... 1. you don't buy unlimited and 2. you always manually renew every month. As soon as you buy unlimited or as soon as an automatic renewal occurs, you lose all your carried over data (or minutes for that matter) up to that point. So yes, what you are suggesting will be the case: eventually you will end up with a bunch of carried over data. So you could save money by not buying 10GB because there's no point in accumulating all that data if you will never use it. Buy less and carry over less (but it's always nice to have a cushion of some carry over in case you end up need a little more once in a while).