You should not have any trouble finding flights on United that you can upgrade with miles or 1K confirmable certs (they are transferable) from SEA-MCO. Your status with United won't hurt you as long as the upgrade space is available and you claim it at the time of the reservation. Make sure you reserve flights with upgrade space available before you buy the ticket. Just tell the agent that you want to reserve flights that have space available on all four segments to upgrade. Since UA has 767s and 757s on that route, it should not be hard to find them as long as you are planning a little in advance.

Good luck and let us know how it turns out!