I know you only want to change the outbound. The workaround would be to reprice both legs - selecting an alternate return that has space either on the same day but different time or a different day. You can keep your fingers crossed that your original seats on the return flight are released and you can follow-on with a second reprice. I would imagine the price will likely be worse that your first reprice since you'd be looking to grab the last remaining (assuming there were freed up) 3 seats.