I had been working on the Mech Warrior in Maya 2008 and my file was saved as a .mb. I tried to open it in 8.5 and clicked on options and indeed it did say Ignore Version. I checked it and 8.5 opened the 2008 .b without any errors. appears it works, at least for that file.
Challenge me and I will try. Because If I should fail, failure also has a prize (Knowledge).