Yes, no jailbreak needed. Same procedure as a update copied via USB.
They are here:
https://pgaskin.net/KoboStuff/kobofirmware.html
Scroll to bottom. Links go to Kobo server(s).
Issues
Don't install an older version than the hardware revision of the model was released with. There seem to be two versions of Sage with exact same name but HW revision.
You might also need to rebuild the database.
Some older versions have particular bugs or lack certain features such as PDF crop and later saving crop settings per PDF.
Sage, Elipsa & Elipsa 2 have the notebooks and advanced notebooks as practically a different system, so on those models I'm not sure how well it works going back.