Garmin etrex user need help changing map ID Number, perhaps w/GMapTool?

Garmin User finds self-combined maps AREN'T visible on his Garmin device after installing, and the Map ID number may be to blame. How can I change the numbers with something like GMapTool?

I've downloaded tons of maps from bbbike.org, and combined them. Ususally, I'm trying to use OSMcombiner to combine both a layer that includes road/folliage/water (like Garmin Openfeitsmap) with a layer with topo lines (like Garmin SRTM Contours).

These maps almost never install nicely on my device, even when they looked good in Basecamp on my PC. And if they install nicely, they don't stay visible on the device for long.

Now, I've realized that since I keep making maps from the bbbike.org source, that they likely all have the same Map id number - and the numbering conflict prevents them from displaying nicely on my device. At least, this is what I'm blaming after 2 years of failure at trying to create a nice, hikeable topo map.

Can someone help me master how use Windows Software to re-number a created map, with something like GMapTool?

Or if anyone has a guide to create a usable combo of those two layers (roads/foliage/water & Topo), I'm all ears.