Besides the pots, the HSS versions have a resistor wired to ground across the lugs of the switch where the singles are wired to reduce the load.
That said, you don't necessarily need 500k pots for the humbucker, it'll just be a tad darker. In your shoes I might look for a humbucker that works well with 250k pots. Or ... another option is to find a single-coil sized humbucker, then you can keep your existing pickguard. For example, I have a Dimarzio Chopper in the bridge spot of my AL with 250k pots and it sounds fantastic. (I went down that same route ...)