If a state changed its laws to take away felons right to vote and applied it retroactively to already convicted felons it would be an ex post facto law.
There is nothing unconstitutional about making removal of voting rights an additional punishment for felons before they are convicted.
You do realize that's exactly what happened with the Gun Control Act of 1968 WRT firearms, right?
It shall be unlawful for any person to sell or otherwise dispose of any firearm or ammunition to any person knowing or having reasonable cause to believe that such person— (1) is under indictment for, or has been convicted in any court of, a crime punishable by imprisonment for a term exceeding one year;
There is nothing unconstitutional about making removal of voting rights an additional punishment for felons before they are convicted.
Ah, so we throw out the presumption of innocence? Or are you trying to say that because there aren't any
people who were convicted but still serving when it was passed that it somehow became magically Constitutional?
I'm sorry, but there's no statute of limitations
on a contraconstitutional act, it doesn't suddenly become constitutional because time has passed.