We tried a number of different approaches, but it failed when it came to how to tell the old documents that they should use the new form in the library. An entry on the InfoPath blog gave a lot of valuable information. The re-linking alternative didn't work as expected, so we turned to the the PIFix-alternative. That was a success.
I'm not sure why re-linking failed, my guess is that it failed because our old documents were using an installed schema, so they weren't pointing to any template URL. Perhaps SharePoint should support that scenario? It might also be unrelated, but I didn't spend more time finding out the cause.
To migrate the forms, we have done the following steps:
- Use InfoPath 2010 to open the 2007 .xsn file and convert it.
- Update the forms submit options so that when user submits, it ends up in the new library.
- Set versioning information the way we want it (we use a yyyy.m.d.n scheme to make it simple).
- Publish the form to the new forms library.
- Create a "dummy" test file with the "+ Add document" button in the forms library.
- Open that dummy file with a notepad application and note the version and product version info as well as the url to the template inside the forms library (it ends with /Forms/template.xsn)
- Map a drive on our computer to the form library.
- Install InfoPath 2003 sdk, you can find the link in the blog article referenced above.
- Open a command prompt and go to the forms library.
- Run PIFix tool from InfoPath 2003 SDK as described below.
Here is a detailed example on how to run PIFix. /v is for the template version and /prv is for product version, the lowest version of InfoPath you support for this form. For InfoPath 2010, this value is 14.0.0, if you set it, you cannot open it in an old version of InfoPath anymore. The /url parameter sets where the document will look for its template. All these values can be extracted from the dummy-file we created in the steps above. I've used Windows Explorer to map \\sharepointserver\sites\mysite\my form library\ to Z: before I start doing anything on the command prompt.