I saw the prompt as well but ignored it as have 3 lots of routes all done and didn't want anything tampering with them, including me. Think I will wait until end of the year then update, as long as the old version keeps working can't see any benefit.
Looks like the prompts to update Express are a Mac thing, I have never seen one on any of my Windows machines Pity because it would save the frustration of finding an update not working. Updating your maps is not going to mess with your routes though. Unless, of course, the roads have changed, but then wouldn't you rather know that before you go. I keep updating my Garmin maps because it simply makes sense to us the most up to date maps available.
John