Thanks, Colin! Great advice!
I just migrated 3 sites from a partner service to our own server. Two of the sites migrated with no problems, but the third site had been originally upgraded from 1.9. First the menus wouldn't work, which I had to click to use online YUI libraries to fix it, and then I had no html editor anywhere in the instance. Your advice saved me tons of time. It worked flawlessly. I used a tinymce file from one of the other instances.
Thank you for sharing your knowledge! I am sharing to let other's know this does indeed work.