If you have a license for ArchiCAD20, I think you should be able to download and install the older versions of ArchiCAD and do a very annoying save back to ArchiCAD17 by going back to each version. Just check your account on myarchicad.com
I'd probably start by setting up the ArchiCAD17 library on your ArchiCAD20 project, once you have downloaded and installed it, and see what kind of problems that might give. Any custom objects need to be put in the embedded library and saved back along with the pln on each version and you still might run into some problems with GDL compatability.
Surfaces will be interesting once you go from 18 to 17, due to no cinerender.
Erwin Edel, Project Lead, Leloup Architectenwww.leloup.nl
ArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5