I'm not the expert here, but I'll give it a try...
I'm quite sure that an MC12 library is NOT compatible backwards so it will NOT work with MC11 or older. So there is no chance to get your new MC12 library working with MC11.
A workaround for you may be this:
Create a new smartlist which contains all your files. Just set a rule "Media Type" "is" and select all available values. Right click on the new smartlist and select "export playlist". In the upcoming window choose "MPL Playlist (Media Center only)" and "Selection" as output range.
If you now import this mpl Playlist to MC11 (I hope this works...), you'll have all your files back in MC11 with all the tag-infos. But you'll lose all Playlists doing only this. If you want to save your playlists too, you should the things above PLUS do a second Playlist export but set "Output Range" to All.
To be sure to lose nothing and to be at the save side, you should do a library backup before you uninstall MC12!
Hope this helped...