Can a UAV of considerable size replace this mission?
Part of the MMA concept is the Broad Area Maritime Surveillance (BAMS) UAV, with about 50 having been planned to be operated from either shore or from an MMA, to do open ocean surveillance.
Note that the military is moving as quickly as possible to unmanned tactical aircraft (see Nav Log article).
the orion is a cornocopia of sensor systems. it's complimented with equipment which allows it to detect, track and kill adversary subs, perform drug interdiction missions, maritime surface surveillance, airborne command and control etc. the fuselage is crammed with avionics from the flight deck all the way back to the MAD boom. certain things that are displayed by the equipment require a trained human element to decipher. I think a UAV application would be just as costly to design and develop.
The best option in my opinion is to reopen the lines and build new airframes and remove the existing equipment from the tired aircraft to compliment the new ones. most of the avionics are just a few years old even though some of the airframes are pushing 40.