The only way you can really find out what will work for your snakes is trying it out yourself, because each snake is an individual. Generally, if they have a good feeding response it can be a bit easier to switch them to f/t.

I recommend buying a couple frozen mice of whatever size they're used to eating live and try a few different tactics (pre-scenting by heating up the thawed rodent with a hairdryer near the cage gets them really worked up so they might not even notice that it's not alive) until you get them to switch to the f/t prey before you try to get them onto rats, because throwing too many changes at them all at once can really turn them off feed. (Rats have a different scent, so it's sometimes difficult to get them to switch onto rats from a mouse diet).

Remember, you just have to keep trying, but make sure that if they don't take the food for an extended amount of time and they start dropping too much weight, just give them live for a while. If you continually try to make them eat and they don't eat it'll stress them out, so it's best to start early and take it slow.