The data is stored separately from the form template, so updates will not
affect the data in your user's form. Updates actually happen before the form
is opened (that's the download dialog you may see when opening a form). And
updates will always happen if the user has network access to the form
template's location.

Of course, if you change the schema in your form template, the update may
break the users form. But InfoPath will generally warning you while
designing a form when this might happen.

Brian