What happens if I go over 100 submissions on the free account?

I am doing a large email blast, and I hope to get more than 100 submissions. If that happens, can I upgrade at that point to the paid service without the submissions starting at 101 being lost?

The form would become disabled the next day of reach the limit. This is for you to give you enough time to upgrade. If you never upgrade, the form will become disabled and nobody will be able to submit anything.