It is not that you cannot but that you can only go down 1 version at a time... The issue is though that by saving down you will be losing information. Whether this informaion loss of relevant or not is a different question, but one that becomes more relevant the more versions you are saving down.
AC22-23 AUS 7000 Self-taught, bend it till it breaks. Win10 | R5 2600 | 16GB | GTX1660
You don't "lose" information, it is that since previous versions does not have the upgrades AC needs to "convert" what the previous version doesn't understand into something it can. IE AC16 introduced the Morph tool so when saving down to 15 all those Morphs had to be converted to Library Parts before creating the file. So IMO it is obvious for GS to just save back one at a time since they don't have to review/reprogram how to jump back to multiple versions. User wise if you need to go back multiple versions it means that something went seriously wrong with your project and it is wise to spend the time checking.
If as a user you just want to save back so that one user works in a new version while the other stays on an older one, GS doesn't force you to upgrade the old version since it doesn't matter which version you buy you can download any of the previous ones.
This is way more generous than the competition and very considerate to its users.
eduardo rolón AIA NCARB Another of the forum moderators. Macbook Pro M1 Max 64GB ram OS X 10.XX latest AC25 US/INT -> AC08 Puerto Rico, BVI, Miami Vectorworks 2023