According to all documentation I could find, and animated conversation with those directly involved with the gnupg project, the commands needed are as follows:
Not a typo. Seriously, "do nothing" was the only answer offered, and it was repeatedly reiterated. And yes, that's when transition from 1.4/2.0 to "modern" 2.1/2.2 was included.
I don't buy it, either. But just copy ~/.gnupg over and see if it "just works". When it doesn't, you can try posting back. The gpg and libgcrypt versions reported by gpg --version might help, but I doubt it.
It's very seriously scary to have so much data locked in for so many users with so little documentation available, most of what's there wrong, and most of all so little sense, or, frankly, ability exhibited by the GnuPG project. None of this would have become an issue at all if they knew what versioning or planning are at all, or at least had a vague idea of what "upgrade path" might mean.
Hope it works as advertised for you, because after that there is only straight-up guesswork and trial-and-error. Nothing else is available.