MC12 can convert an MC11 library without problems. It is not possible the other way around. MC11.1 cannot read an MC12 library.
The easiest way to convert an MC11 library is to create a library backup file in MC11 and restore it in MC12
(File> Library > Back Up / Restore Library). All file data, user fields, view schemes in the tree and playlists will be preserved.
The "Customize Current View" options (sorting and column configuration) will be resetted to default because of the internal changes, but these are relatively easy to regonfigure.
The program options
(Tools > Options) are not included in a library backup file and you need the set these separately. You may want to check what you have set in MC11's Options and make some notes.
If you have more than one library create a separate backup file for each library. In MC12 create first the needed additional libraries and restore the backup files individually.
You can keep the latest MC11.1 build installed (MC12 may not work correctly with older MC11.1 and MC11 versions) and evaluate MC12 while still maintaining your old library with MC11.1. (The last MC11.1 build, 11.1.201, is available here:
ftp://ftp.jriver.com/pub/downloads/music/old/MC111201.exe.)