>Posted this originally under Disease forum and had no response so I'm trying it here.
Sorry. Between work and raising queens, I haven't had much spare time lately.
>First, is it necessary to switch out all of the comb at one time or can I just insert a frame or 2 when convenient as other old comb comes up for replacement.
You can do either. I would feed it in. Between what some people do as normal replacement and what I would do for swarm prevention, you should be able to swap out all the comb in a couple of years if you're agressive, and at least all of it in five if you not very agressive. The problem is it will probably take two complete turnovers to get down to 4.9mm or less.
>Second, is it a tremendous stress on the bees to go straight to smallest size foundation or should I first raise a generation on 5.1?
You can give the bees 4.9mm or 5.1mm and they will still build 5.1mm the first step. It really doesn't matter which one you do.
Personally I'd just make foundationless frames or use 4.9mm starter strips. After a generation of smaller comb you can go to foundation if you like, but you may find you like natural comb better anyway.