Konablack is right, the frame crank arms of the restored bmx might be longer than what's intended from the frame. or it's a frame designed for a larger wheel but a 20" bmx wheel was installed.
anyway, back to your question. is there a learning curve, yes but it's not steep. you have learned how to balance, steer and pedal, so that's pretty much it. what you'll find different are the following:
1) gearing - you'll have options now compared to a single speed bmx. try to maintain a pedalling rate of 90rpm regardless of the terrain. your gears will help you achieve this. if you don't understand what i'm saying, watch lance armstrong, he pedals the same rpm regardless if he's climbing or on flats.
2) torque - larger wheel, more force to pedal if you're on the same gearing as your bmx
3) stability at speed - physics will keep the larger diameter tires running straight at speed. with this in mind, it's more difficult to steer it when you're really going fast.
4) toe overlap - let's see....the early 29'ers used to have toe overlap. the 26 and 27.5 are not prone to this. if you have good body english when turning, you might not encounter this on a 29'er.
5) wheelbase - turning a tight turn will require a larger arc since you have a longer bike.
curious question: you said your slippers hit the ground when turning in your restored bmx. how do you position your inside foot? sample, if i'm turning left, my inside foot is my left foot. i would position my left foot at 12 o'clock and my right foot at 6 o'clock.