An assistant to Jon Pertwee's Doctor from 1971 to 1973, Jo (played by Katy Manning) is due to reappear in a forthcoming episode of The Sarah Jane Adventures.
"I did go back and watch the end of [her last story]," Davies told SFX. "I think it's really important to preserve the absolute beauty of what [producer] Barry Letts and [script editor] Terrance Dicks did there."
Jo parted ways with the Doctor in order to explore the Amazon jungle with new fiancé Cliff Jones (Stewart Bevan).
"I know there's a feeling that she'd have got divorced [since] but that's absolutely betraying the way we left her," argued Davies. "We left her with the promise of happiness, of adventure, of love and joy, and you can't be cynical and undercut that. That's just interfering with classic Doctor Who and rewriting what we were promised."
The Sarah Jane Adventures will return to BBC One for a fourth series in October.