Hi Louise,
You must have added new fields that were required (or deleted fields that existed in the old form). You can do a custom upgrade or just publish as a separate XSN (and keep the old XSN for the old XML forms). If you need help, we could look at it for you, but we'd need to receive a support incident retainer in advance. You can go to the Support page on Qdabra->Products page and purchase a support incident and we can schedule some time to help you. Depending on how many changes were made it could be a simple change to add custom upgrade, or more involved.
The key to backward compat is to only add non-required fields and never delete or change a field name.
Cheers!