I'm guessing you meant to reference GeoMedia 2015 rather than 2005? Assuming this, then you are jumping several major releases (2015 - 2016 - 2018 - 2020). Though in most cases historically there is minimal impact to commands even with a major release, there certainly are some that you need to consider here especially when leaping forward 5 years.
For example, with the 2020 release we upgraded to use Visual Studio 2017 - you will need to rebuild your custom commands and retarget to later versions of the .net framework and C++ runtimes.
With the 2018 release we had to change how some of the coordinate system and raster technologies were deployed, that are shared with other Hexagon Geospatial product lines. So if you use API from these systems, you will need to make changes to ensure they are properly discovered by your commands (whether programming against COM or .net API).
Details of such impacts are always provided in the "Developer Impacts" section of the Overview chapter of GeoMedia Object Reference, available on the Start menu. - Hal
