I think different definitions of retroactive are being used.
If the prohibited persons list has to do with possession, no retroactivity is needed - you get put on the list for the crime, so you can't own it anymore.
What happened is that someone was convicted of a crime that wasn't on the list, so they got to keep their guns. Then, the list of crimes was amended to include what they were convicted of, and that was done retroactively - not just to new convicts.
It seems to me like if they can add crimes to the list that they're increasing the penalty after the crime (aka retroactively increasing the penalty), which is something I'm opposed to. Once you're sentenced, as long as you obey the terms (probation etc) it shouldn't get worse.