The best way to get them to switch is to just give it a try. You might find that your snakes don't really care (especially your Boa). However, if they don't take them there are a few things you can try.

First off, you can try scenting the room for about 30 minutes before trying to feed them. You could try stunned, but I believe the largest obstacle is to get them to eat something that is being offered to them on tongs instead of just a scent difference between stunned and F/T.

Make sure the F/T rat is warm. Hold the rat behind their shoulders with the tongs and slowly approach your snake from the side. Boas and Pythons are ambush predators so they will feel more comfortable with a rat going by them instead of just being wiggled in their face.

Oh, and the CO2 chamber is definitely the way to go.

Good luck!