ArchiCAD 14 (and 15) can read files from versions 8.1 onwards:
http://www.archicadwiki.com/ArchiCAD%20versions
Nobody should have files in 8 format since 8.1 was a free upgrade, basically fixing tons of bugs in 8.0 But, if you do actually have an AC 8 file, you'll have to download a copy of AC 10 or 11 to do an intermediate conversion.
All of the steps to migrate a project are in the beginning of the AC 14 Reference Guide PDF in the chapter "Migration Guide for ArchiCAD 14".
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Ventura 13.7, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB