About upgrading forms
When you modify a form template in design mode, the existing forms (.xml files) that are based on it may need to be upgraded so that they are compatible with the modified form template. This ensures users can continue to work with the existing forms. If forms are not upgraded, they may not open or function properly, and data from existing forms could be lost. When upgrading forms using Microsoft Office InfoPath 2003, you can either automatically upgrade existing forms or create a custom version upgrade using script.
Choosing to automatically upgrade existing forms when you modify their form template helps to ensure that changes you make to a published form template don't prevent users from opening and working with existing forms (.xml files) that are based on it. Choosing to automatically upgrade existing forms also allows users to open the existing forms, even if the form template's XML Schema has been changed. This is because InfoPath is able to schematically reconstruct the .xml file, so that it is compatible against the modified form template's new XML Scheme.
Choosing to create a custom version upgrade of your forms using script can be used when you need to handle more complicated upgrade tasks automatically. When you create a custom version upgrade using script, you can immediately edit the script file in Microsoft Script Editor (MSE), and then update the form template with your custom code for handling version upgrades.