Howie, the Garmin updater should be smart enough to know if you already have the current version or are replacing an older version, just follow the updater's lead and you should be good to go.
Only issue you may have is that the old handheld might not work with it (the map app) unlocked for the Zumo... I have a Garmin 60CS that I used before the Zumo and I installed the new Navigator the other day when the Zumo arrived. Have not tried to use it with my 60CS...but I have a feeling it won't work...have to try it this weekend.
Garmin is a PIA with their unlocking crap. I have an account on the Garmin page that has all of the keys that I have used over the years...helps when I rebuild the PC's and need to unlock the software again.
Sean