first off, are you talking about detransitioners? Trans men who later reidentify as women? Or are you talking about transfeminine people, (in which case they arent men at all) ?

Yes, if someone is out as a lesbian woman and later discovers he is a straight trans man, that is sexuality changing. This also happens to people over time, whether caused by hormones or just fluid sexuality. People can identify different ways at different times in their lives as they learn about themselves, for example a gay man may discover he is bi a few years after coming out when he meets a beautiful woman he is attracted to. Sexuality can change.