Wonder what your old mechanic is talking about with the can of worms. Shaft tubes are removed and replaced fairly easily by most competent mechanics. The stuffing box and coupling hose need to be removed. The tubes as pictured in this thread, are held in place with 2 or 4 bolts which are removed, and then you just break the caulk loose. I get my work done in a commercial yard that caters to Chesapeake Bay waterman. The yard pulls 1 or 2 stern tubes every month. Most are on fiberglass boats, but a fair number are on wood boats. These boats can run in access of 2,000 hours a year in the shallow waters of the Chesapeake . Replacing cutlass bearings is a common occurrence. In many cases the shaft isn't removed, they just pull the prop and then the stuffing box is removed. They do an inspection of the shaft tube and hardware, replace the cutlass bearing, inspect and service the stuffing box and connecting hose as necessary, and reassemble. Haulout, service, and back overboard usually the same day. Pulling a stern tube is really no big deal if you know what your doing and it's a serious commercial yard.
Ted