transitioning from live to frozen thawed?

hello everyone! i just took in 2 BCIs that i’m fostering that were cohabbed in a 50 gallon and were being fed 2 medium live rats a week EACH. they are both extremely overweight. i plan on only feeding them every 3-4 weeks one small frozen/thawed rat each. i already have my own costa rican BCI in a 5x2x6 bioactive enclosure and he gets fed every 1.5-2 weeks and looks perfect weight wise. anyways my question is how do you guys think they are gonna transition from being fed live to frozen thawed? i have never fed live before and never want to and have never had to transition a snake from live to F/T before. do you guys think they’ll take F/T right away? especially since they’re gonna be extra hungry after 3 weeks compared to how often they were being fed before.