geoffc:
I am currently developing a couple of InfoPath forms for use in SharePoint. Our users brought up a question that I was not too sure about. They wanted to know if InfoPath pushes out changes to the original form template to already created documents. They are thinking that after the form is released, they will probably need to make some revisions to it, however, they do not want the users to fill out a completely new form. Any info. is appreciated. Thank you.
InfoPath handles upgrades automatically, as Shiva said you can write code if you want to manage the upgrade process and in some cases it is required that you use your own upgrade code to perform the upgrade of old XML's (when you make schema changes for example). But in many cases InfoPath will do this automatically.
You users existing forms should be upgraded in most cases without issue and existing data would not be lost - they will just have the new fields there.
here is an article you might find helpful in explaining the options in InfoPath to do this:
http://my.safaribooksonline.com/0735621160/ch08lev1sec3?portal=oreilly
hope that helps