If a user has an existing InfoPath form that was created with a template, and then this template's form ID changes (and that's all that changes), what happens to the user's form? Is it still compatible with the template? Does this invalidate any digital signatures on the form?
The form will still be compatible, and the digital signatures will remain in tact. (see caveat at the bottom)
The user's form (an
.xml file) contains no references to the form template's form id, so it's none-the-wiser. The form id is used, however, for caching purposes on the local machine. Your computer will check its cached versions of the form for that form id before downloading the form template to open the user's form.
Depending on your InfoPath settings, however, performing this change and saving the form template over itself may automatically update the form's version number. And then, based on other InfoPath settings, this change may prompt the user when they open their form to update to the latest version. This means that if you're determined for your user to remain completely unaware that anything has changed at all,