It depends which component of P6 they are talking about.
The P6 desktop client could be upgraded (or re-installed).
The P6 database would always just be upgraded
The P6 web components (P6 Web, TeamMember and Progress Reporter) are pretty much always uninstalled and re-installed between both major and minor versions (service packs and fixpacks can be applied in-place)
*From the P6 version numbering of say 220.127.116.11 (the most current) it breaks down into Major Version(8) . Minor Version(3) . Service Pack(1) . Fix Pack(3)