So what I'm reading is that I bought/upgraded to V17 and that upgrade service is due to expire soon.

I've just upgraded to V18 and it begs a question like @EuropaViewer - If you re-install it goes back to your last purchased version.

So what happens after I've upgraded to V18, within the update service period and then have to re-install? The way I'm reading this is that V17 is the best I can get.

But this leads to another major issue in that any files created or saved in a newer version may be crippled if opened in an older version. For example my website, developed in V16 then V17, and now modified in V18 now complains if I open it in V17 with an error dialog of "Warning from Xara Designer Pro X. This file was produced by a different or newer version of the program. It contains some data that will be ignored. Technical details (4650)". So that means I can no longer use my website xar with V17 with any sense of reliability or guarantee of compatibility.

Surely if you updated to V18 during the update service you should be able to re-install the V18 version that you had when the update service expired. If the files are not backwards-compatible then there are serious implications about upgrading to a newer version during the update service period.