How are you using MU for basemap replacement?

You mean it won't downgrade the software back to standard version 4.00 even using updater.exe even if you change the software version of 029201000400.rgn to 421?

What if you use WebUpdater in command prompt? C:\Garmin\WebUpdater\WebUpdater.exe /upload_file rgn=3 path=C:\Garmin\gmapbmap.img -autorun usb /i
That method won't work on my StreetPilots but worth trying.