Key thing is to only add fields that are non-required. Don't delete or rename nodes or add nodes that are required.
Getting the schema right at the beginning is really important. Having field1, field2, etc. is a very bad practice because people have no way of knowing what the field is for and when you start submitting your data to a database and have to do mapping, it is really hard to keep track of all fo the field1s.
We have a Doc Migration tool that migrate forms from one schema to another. You can even use this tool for free to batch migate hundreds or thousands of forms. Just install the free trial of DBXL. Get the Doc Migration tool. Add your XSN template to DBXL. Import all your XML files. Read Jimmy's blog on how to create XSLT mapping files. Create the upgrade XSLT. Setup a new XSN with the new schema. Run Doc Migration from old to new. Export all the XML from the tool. Presto!