Its contractual. They can’t stop it. There is a check block on the Displacement Bidding page to select if you want to volunteer and displace into a different category. The only caveat is the person you are displacing has to be able to hold it (FYI you are always taking the displacement of the most senior person being displaces). Here’s an example.

You are an Airbus Captain in SFO and your seniority is 6,700. You are going to be one of the most junior Captains when this is over, and be on reserve for sure. You decide to just take another Captain’s displacement, and go to the 777 FO instead. Perfect for someone who wants a paid move and get a paid move and positive space travel. Or you just upgraded from FO to Captain, and you commute and don’t want to be on reserve, so you’d rather go back to senior FO. I guarantee there will be people that volunteer.