converting those launch pads from a Saturn V to a Shuttle configuration cost several hundred million dollars. They had to change just about everything. The launch tower is different, the fuel and electical interfaces are different, the thrust deflector is different, etc. They also had to completely redo the support/ground equipment, launch control computer systems, and a host of other important details. It might've been easier and cheaper to build new pads from scratch than to convert the old ones.

Going from a Shuttle to a HLLV derived from Shuttle technology shouldn't be as difficult. You'd need to change the thrust deflector to handle engines mounted on the bottom of the ET instead of on the side like the Shuttle. The HLLV propellant interfaces would be mostly the same as the Shuttle except for the upper stage. Most of the electrical interfaces wouldn't change radically from a Shuttle to HLLV. You might not need to radically change the support equipment except for the really old stuff (I've read that some Shuttle ground systems are still based on 8086 technology). Still, we're talking about NASA. I'd wager converting those current Shuttle pads to an HLLV pad would still cost at least $100 million.