The BBC said Saturday that Davies is set to end his run as the series' producer and Tennant is being replaced as the show's beloved Time Lord character following the final two "Who" episodes.
"I'm very proud of these two episodes," Davies said. "Now I simply feel happy to be honest, really happy with what we made."
"We've got one or two surprises in store before the episode on New Year's Day -- and it might not be quite what you expect."
Actor Matt Smith will take over the "Who" lead role from Tennant, while Steven Moffat will assume producer duties from Davies.
Davies, who helped "Who" return to television in 2005, told the BBC he remains in contact with Moffat regarding the everyday production of the popular science fiction series.
"He does find time to e-mail me every so often saying it's all a nightmare, and he's never been happier in his life," Davies said.
"I've read some of the scripts -- they are beyond brilliant -- I can't tell you what thrills and darkness and comedy you've got to come."