briang2ad, I mentioned this in a similar post that you had.
You can make 204 cases from 222 Rem Mag cases by necking the 222 mag case down to form a false headspace shoulder, then blowing out the case in a case forming firing of that case.
The 204 has a longer case shoulder by .075 and it is very difficult to move that shoulder back without wrecking the case, unless you have special case forming dies to make 222 ReMag cases. It's not a simple matter of running 204 cases through a 222 ReMag die.
IMO, the effort is not worth it, especially with 222 Remag cases still readily available.
NCsmitty