You might want to downgrade your domestic upgrade if your original booking class is still available, so that you do not run into wasting your SWU. I would recommend you do so every time you apply your SWU - you might need to call United to have an agent manually do it for you as it seems that the recent change in United website does not allow segment by segment upgrade. When your long haul is cleared, you can ask United to extend your SWU to the domestic portion.