The ability to upgrade was only available for songs purchaseable as DRM-free on iTunes. I still have tracks in my library with the DRM because the distributor went out of business, making it impossible to upgrade.
On Linux nowadays, but last time I was on macOS with iTunes I has an album of Australian folk rock that still had DRM, were still an old bitrate, and not upgradeable to "Plus" (yes, the thing from 2007) or DRM-free.
I think most were but also as I mentioned elsewhere on this page, Japan and possibly other countries didn’t go DRM-free for some time so possibly they had iTunes Plus but with DRM.