Difficult

Hi! If you want to have the two OSes on the same partition, it is somewhat difficult. I remember having it that way quite some day ago. With the help of a tool called "OS Switcher", it was possible to select the active system folder. But I would definitely suggest having two separate partitions, one for each system folder. If you want only one system folder and your problem is simply downgrading, I have no idea if there is any risk. If your computer has a CD ROM drive from which the Mac can be booted, it should be ok to wipe the 8.1 off the drive and then install 7.5.3. But don't do such things if there is a slight possibility for ending up with no drive to boot from. best wishes, Oliver S.