The argument is that you are supposed to sacrifice you're own life rather than convert in these situations. These forced converted Jews broke one of the three commandments (idolatry, adultery, and murder) that you may never break under any circumstance even if it means giving you're own life. So some say these Jews do not have place to convert back while others understand the difficulty of the times and maybe that some just converted publicly and still stayed Jewish behind closed doors. Each his own opinion and up for debate