I'm not sure this would hold up to scrutiny, as the office of Supreme Court Justice and federal judge are different offices. The extreme version of this is that we'd obviously not get away with interpreting their office to be "civil servant"and shuffling them off to the DMV.
Different offices, but the process for a Supreme to retire and move to "normal federal judge" already exists - no re-confirmation or other antics required.
Of course, SCOTUS would end up being the ultimate arbiter here, so quite likely a non-starter for that reason alone.
