So do you need to uninstall the mc20 once u completed the installation of mc21?
as AwesomeD said already no you don't .... actually I recommend to the people I have helped install their systems with MC that on a version change they do NOT uninstall right away ... at least until the next version is up and running correctly.
what I do
1) make a back up of the prior versions library and name it specifically (like justb421 or something); if wierd crap happens you can do a clean install of the next version and use that file to restore directly. If the user plans to reorganize their hard disks or filepaths do it BEFORE they upgrade or after .. NOT in between (eg. uninstall version 20; change to a new NAS or move your file structure) then do a clean install of 21 is going to pose a bunch of problems ... usually solvable, but complicated when it doesn't have to be. Just saying as someone recently did this and was not a hapy camper
2) go to options in the prior version (20) and make sure the any options to run JRiver on boot (either the main program or just media server) are set to none
3) remove any shortcuts (like in the task bar or starup scripts that would launch the prior version by mistake)
4) install version 21 and ervything will be (should be^^) ported over automatically no sweat, no restores, no re-config. Set up windows boot options if you use them and/or any shortcuts if needed and get your licence sorted
So IMO, then, and not until everything is copacetic, you can remove the prior version - (there can be some issues with uninstalling, if so reinstall prior version and then uninstall .. this happens to me as I always keep 2 versions to help others who don't make the jump, so I might have three versions on my machine at one time ... this won't affect most people.)
of course if you have the library backup file you can uninstall the prior version install the next and restore the library, but I find this is the most trouble free way of going about this. This also permits you to go back and use version 20 if initial releases are not as stable as you'd like.
just my two cents of course. hope this helps