^ (two posts up) Creativity and ingenuity will fix those problems. Also, you have to think bigger. Those are issues because they'd have to cram everything into 75 minutes. Spread it out a bit!
The actors looking different is a non-issue. A normal real-world explanation that everyone can relate to and a valid onscreen explanation has already been presented. And, The Five Doctors had a notably older Pertwee and Troughton, and it was no problem at all.
What you describe as a "false tradition" is pretty much how traditions are started! You first experiment, people like it, and it continues!