My upgrade to MC21 went just fine on both of my machines. Everything was already there, all major settings copied from my MC20 install. The only thing I had to do in both PCs was to point my library again to Dropbox - by default MC21 install had reverted to a local library.
Brian, I'm sorry but I have to disagree with you concerning the speed of MC when it uses a library from inside Dropbox. I see absolutely no speed issues at all - which should be the case, since Dropbox isn't actually a "network drive". It is a local folder which is kept in sync with the network folder in background.
I have monitored this for a long time now, and seems like whenever I stop using MC and quit it, the files to sync with Dropbox are 16 small files, like mediafiles.jmd, curplaylist.jmd, index.dat, user.jmd etc. The files are very small indeed, and it takes about a second from Dropbox to sync them.
Having all this syncing fully automatic and happening on background is a huge advantage to my work. At the moment, I am creating new playlists for my classes on my desktop PC. After I quit MC21 on the desktop, it takes a few seconds, and my new playlists, special views etc. are in use in my laptop (which I use to play music on my classes).