There is no need to do a full reconvert on those guest, just move into a dummy domain, Then add it back to the correct domain, if these guests have different Computer names then they will add to the domain as new member servers, as side effect of this is that they will get a New Domain SID too.
If as I supect they actually have not got a new computer name then you will need to run sysprep on the two guests to make them unique. this will change the local SID, and Computer name, then once complete. re add them to the domain.