Hello,
You need to do in the following to upgrade version automatically.
1. Open form in Designer mode and then Go to Forms ->'Tool Options'
2. Go to 'Advanced ' tab
3. Select 'automatically upgrade existing form' value from "On Version Upgared drop down box in "Form Template version" section
4. Click 'Ok' and save the form.
Make sure that old version number and new version number difference is more than 50.
Still you are not getting then you have re link those documents in SharePoint.