You are smart to ask before making changes. :)
In general - don't delete, move or rename data source nodes. If you change the validation for a node, consider the impact on previously created XML (i.e. - you allowed a field to be blank in the past & now it can't be, what happens to the previously filled out XML when they are re-opened?).
I usually set my form versioning to auto upgrade existing, because if I add new nodes, I want to make sure they get added to my existing XML on open. But that is also a place where you need to consider how existing XML should behave.
Here's a blog post I did a while back on previewing a template with existing data. I find this invaluable when making changes, because it lets me know what will happen to the XML when it is opened with the changed template.