MB files are binary files written so only Maya 8 files can be opened in Maya 8. However Maya 7 files can be opened in Maya 8, but once saved the MB file will change and will only be usable for Maya 8.

MA Files are ASCII based and are written in plain text. This can be amended in any text editor and all references to Maya 8 can be changed to 7 or 6.5 etc...

Saying that, if your scene has something in it that is only available in Maya 8 then you may experience issues due to that feature not being available in 7 or 6.5 or 6 or 5 or 4 etc... I would recommend only using one or the other and not switching between the different versions.