Conceivably you can convert Sheppard to LRT more easily than you can the SRT to LRT though. Here's how I envision it happening:
Step 1: Reduce Sheppard trains to pairs (down from 4 cars), and double the frequency. Minor increase in operating costs, but it needs to happen.
Step 2: Wall off half of the Sheppard platforms, so that the subway is operating entirely from either the eastern or western half of the platform.
Step 3: Lower the platforms and associated infrastructure (stairs, elevators, escalators) on the half that has been closed off.
Step 4: Shut the subway down over a summer to convert the tracks, electrical, and signaling equipment to run low-floor LRT vehicles. Run shuttle buses instead. Since it's the summer, demand is less and people don't mind waiting outside as much.
Step 5: Begin work on the other halves of the platforms (lowering, adjusting access infrastructure, etc) while the track and electrical work is going on.
Step 6: Re-open the subway running LRT vehicles on the completed halves of the platforms. Work continues on the other halves.
Step 7: Re-open the other halves of the platforms to have full length, low floor LRV platforms.