[skishmonkey72]

From a 100% pure one-hander: Yes, practice definitely. Also (assuming you're playing index on left, middle finger on up and down, ring on right), get used to moving your middle finger between the up and down keys really fast. And when you get decent enough one handed to do faster songs, do those songs over and over again (something like Max Forever 4 or 5 times in a row) to build stamina. If you don't have stamina in your middle fingers, you'll have a hard time getting past Difficult songs one-handed. Also keep in mind that your fingers are learning new reflexes, different from the ones you had playing index, so you have to give them lots of practice time. The more practice, the shorter the reaction times of your new one-handed playing reflexes, and the better (and faster) you can play.
A side note: One-handed play is not particularly strong of a playing style for PA, imo. So don't be surprised if that comes slower. On slower songs I find keeping my fingers off the keys when I don't absolutely need them there (aka. pressing a key) and tapping like the keys of a typewriter greatly improves my PA.

Hope this helps...