Return your Nuvi to Original Firmware.
Use Webupdater to update everything that you need including the Firmware.
Use GarminMapupdater.exe to update maps, Make sure that you select Unit and Computer.
Now Everyting is up to date. Make a backup of the whole Nuvi and Memory Card (If applicable).
Now convert to Zumo.
When in Zumo mode do not connect the Nuvi to Garmin Servers at all.
If you need to update Maps you will have to follow the procedure again.
Take the Backup of Nuvi Internal Flash that you made above and copy it to an empty flash drive.
You can use the Flashdrive to connect to garmin servers.
You can edit Garmindevice.xml to change the firmware level to satisfy the servers about firmware update.