Thanks for all the replies. I got an email back from Streamlight CS and they confirmed that the old style slide switch cannot be replaced with the new style and the chargers are different as well. To me that makes the old style SL-35 a little less desirable. Plus the old style charger seems hard to find. I think I will keep looking for a good SL-35X and pass on the old SL-35.
I really want one with no battery so I can put in a nimh replacement pack. It should have a nice long runtime with the 9500 mAH nimh cells