We currently have a sub account for that we use that we also pay for.

We currently have a sub account for that we use that we also pay for. It seems like everywhere sub accounts are only supposed to be free.

Does that sub account also have forms and submissions stored in it? If you're using the sub account just like a regular main account, then, I believe, it is only proper that you upgrade it. Limits are also applied to accounts even if it is treated as a sub-user account.