I think, that as long as you have the proper software update for a unit, it doesn't matter how old the unit is.
Make sure you back up any waypoints/tracks/routes to an SD card before you update.
For the GPS part, it may depend if you have an International unit or not.
Those units have a world map in them.
The software updates do not update the stored map, only how the unit
functions. They may also change the GPS receive rate. It might be possible that your GPS receiver may not be compatible with a newer update, if the receiver is too old.
I would call Customer Service and give them your unit's serial number just to make sure it's safe to update.
Keep this in mind: You now have a working machine. If the update goes wrong, you may have to send the unit in for repair.