When you migrate a form library from Sharepoint 2003 to Sharepoint 2007 references to the Form template will still point to the 2003 form library.
If you need to update the reference Sharepoint has a relink documents feature to individually re-link the xml forms to the appropriate template.
Unfortunately this process can be rather cumbersome especially if the form library contains sub folders. To alleviate this painful process
I developed a tool that with go through an entire directory structure recursively and do a find and replace on the template (.XSN) file.
What you will need to do is go into the explorer view of the form library, copy all files and folders to a location on your local hard drive.
Then open up the “UpdateInfopathTemplate” utility and update all the xml forms. Then copy all the forms back to the destination form library in 2007.

Click here to download the utility