The Migration Manager works fine for moving from 7.5.4 to 8.x, but the backup/restore tools were no succes when I tested that. The tools and backup format changed when 8.x was introduced. A later version (8.1.something?) introduced a tool that supposedly is able to convert a 7.5.4 backup into something you can restore on Plesk 8, but that didn't work when I tried it.