A “save as earlier version” function does not exist in Dorico. Any version of Dorico will open a file from any version of Dorico: you can open a Dorico 3.5.10 file in Dorico 1.2, for instance.
The only real consideration is that if you’ve used functionality in 3.5.10 that doesn’t exist in Dorico 1.2, when you open the file in Dorico 1.2 it won’t display those elements correctly. If you then save the file in 1.2 you may lose the 3.5.10 bits that had previously been saved.
There is no such problem with the three different flavours (SE, Elements and Pro). If in SE/Elements you open a file that has more players than SE/Elements can handle, it will open as read only (you can print/play/look around the file but not edit it). If you open a Pro file with fewer players than the threshold for SE/Elements you can edit the file as usual but with SE/Elements’s restrictions. Saving a file in SE/Elements will not delete any Engrave mode data, for instance - you just can’t actually change that stuff in SE or Elements.
I’d argue that this is a better system than Sibelius:
With Sibelius’s system:
You lose the bits of the file that are recent-version specific at the point of saving the file.
The operator of the earlier version can’t open a file from a later version without either updating or instructing the collaborator to proactively Save As Earlier Version.
With Dorico’s system, neither of these apply; if you open a 3.5 file in e.g. 3.1 you’ll be warned it’s a project from a later version or Dorico. The only real downside is that you won’t know what’s displaying incorrectly (or not at all).