What is the preferred method for removing & replacing electrical wiring in the garage?
Cut open the drywall. You can then use a piece of plywood to cover the hole so that it's easier to remove in the future.
Sounds like too much work. First step, make sure they are GFCI protected. If you don't know if they are GFCI protected, you can get a GFCI outlet tested from the hardware store. And while you're there, get outdoor box covers. Then you can just replace the face plates with the outdoor box covers. MUCH easier than removing the outlets.
You don't need conduit. Get an "old work" electrical box. Cut a hole BELOW your breaker panel. Run the wire from that hole straight up through a knock-out in the bottom of the breaker box. Then you can push the wire through the top of the "old work" electrical box and mount the box in the wall. See, no conduit needed!




Based on response to my first answer:


Hooray!
Turn off the breaker. Remove the outlets and install wire nuts on all of the wires. Install blank faces plates.
OK, you still don't need conduit. Just drill a hole through the top plate of the wall. You can then run the cable up the inside of the wall, through the hole, and then down to your breaker panel with all of the other wires.