It seems the best way to downgrade is to have the .upx file on his own PC, transfer it to the Max2 and update from the Max2 while telling it the file is local.
I did it twice without problem.
To get the old version, you should ask Onyx to offer them on their site. There is already some people asking for that. You may add your voice.
http://bbs.onyx-international.com/c/MAX2-SERIES