It's a steam limitation, Nothing that TFP can do about this. If you want to keep a copy of each different version, install Version A, copy the entire folder to another location, then install Version B- Version A will remain untouched as a copy, while the one managed by steam is now on Version B.
So you can easily have separate installs of A16, A17, A18, A19, or any combination of them or the ones preceeding them.
Edit: And to note, Strongly recommend making the one you let steam manage be the latest version- The old versions are unchanging, so it's perfectly suitable to copy them to a new location as no update will come for them.