There should be enough storage space on the device to do this, make sure you have a backup of the old map prior to the update as Home will remove the old version as part of the update.
Once the update is complete you can open the .pna file in the map folder with NotePad and change the ID line save the altered file with the same file name then copy the old map folder to the device.
On the device go to Change Preferences/ Manage Maps/ Switch Map and you should see both the new map and the old map listed, the old map will be named whatever you editted the .pna files ID line to read - Mike