You can upgrade until your year of free updates expire. You will keep the software you will have at that given time. Nothing will be downgraded, just make sure that version is a stable one and also it is wise to make a few back ups from versions just before that if you know how to.
I own a MaxiSys908Pro and a MaxiDas708. Never registered the maxidas since I wish to keep my versions as old as possible on that one, never mind asking yourself why, that's something the average user does need to worry about. The MaxiSys has a lot of updates, most of which are good additions.