Tough decision. I tend to think that if a guitar's been left unchanged for a long time, it's sort of bad karma to mess with it - one of the reasons (apart from cost!) I don't buy vintage guitars.
But you're changing the pickups anyway, and they will almost certainly work better with 500Ks than with the old pots. Then again, if the pots are 300K/100K and you're used to them, the brighter, more lively tone with 500Ks might come as a bit of a shock!
And to make matters worse, wiring a 335 is such an awkward job - you really want to make the right decision first time!
If you do decide to keep the old pots, get the tech to clean them while they're out of the guitar - that should fix the scratchiness at least.