In general, you'll want to switch them using the same prey item they're used too. IE, if they are getting live mice, give them f/t mice.

I feed f/t by putting the item (in the case mouse) in a baggy, in some tepid (luke-warm) water, and place it in the cage, with the baggy open wide. Leave it in there for an hour or 2 and that should get them interested (The scent of thier usual item should be enough to get them interested) After an hour or so, check the mouse, if its squishy, then remove it, and place it in the absolute hotest water you can get from the tap, leave it there for 5 secs, and try to feed.

As usual, keep the room dark, and thier hid on them, or atleast near by so they can drag it in and consume

If they don't take it however, don't be discouraged, and keep trying every week at thier usual feeding time. Snakes can go weeks without food, so a week without food isnt a big deal, eventually, they'll tag a f/t like a freight train.