Basecamp/Mapinstall Always reloads all my maps, when trying to add a new one

[nahannidog]

Hi,
I've read the great tutorial about using Basecamp to  load maps to GPS receivers [https://www.gpsfiledepot.com/tutorials/how-to-load-maps-on-my-garmin-gps-unit].  In my case, using Montana 650T with SD card and US Topo 100K in ROM, the process behaves differently than indicated in the tutorial.  Especially annoying are:
1. When I add a new map, no matter what I choose, ALL the maps get re-written (painfully slowly). MapInstall goes through as many cycles of "Building MapSet"  as there are old maps plus the new one.  So "Leave as Is" selection seems to be ignored
2. The carefully_chosen_filenames.img of the existing maps are rewritten with defaults e.g. CO & UT GarmTopo24K.img (1.321GB) becomes  TOPO US 24K Southwest.img (1.321GB)

Or is this the typical experience?  Everything gets rewritten??

Version info:
Basecamp Windows 4.7.4
Latest Montana software version 7.60