Yes, all version of Xara are backwards compatible so if you are thinking of upgrading to a newer version, you shouldn't have any problem opening your files in the new version and continuing to edit them.
If the version upgrade is quite large, for example from version 5 to 15, you may find some things are out of place or perhaps doesn't look quite like how you originally designed it. This is likely due to improvements being made to the rendering and positioning code since the early versions. If so you will have to simply re-position your objects.
What happens if I open my file in an older version of Xara?
You will get a warning about a file being made in a newer version, and any newer features that aren't present in older version will not work, but it should still open. If you save your file in this version and open it back in a newer version, you may lose newer features in your document.