Problem with the upgrade process

I have two membership levels for my site. An issue I found today was that if a member upgraded to the other level they weren't charged but their account got renewed to the new level with a new and improved re-new date (e.g a free years pass).

I have changed the upgrade button in the options menu from 0 to 1 day and this seems to have solved it, but what I am unsure of is if this will stop the users being able to upgrade for free once it gets down to one day?

Thanks in advance
Ken