It's not a promise (can't check since I only have 1 version installed), but it is an idea:
If sketch works like AutoCAD, each version has it's on folder in the "program files"->"Autodesk" folder.
Find the fonts in each folder. Move the fonts from v8 (save them in my docs or something, just in case) and then copy-paste all the fonts from the v7 folder into the v8 folder..