if you have complete outers on the rear gear cable and it isnt threaded through guides you dont have to touch the rh handlebar at all. make sure the rear cable is long enough for the biggest frame.
if you use a matchmaker on the lhs you can undo the two bolts holding the brake on then undo the steerer clamp bolts pull the bars off and the forks are out. you don't even need to take the front wheel out. 2 minutes tops. cut the cable ties for the rear brake and swap the whole rear brakes over. 5 minutes including re cable tieing.
ht cranks, 30 seconds max to get out. use the same headset and keep the spacers stacked ready with the frame. bb already in other frame. i'd keep a front mech fitted on both frames, with the lh shifter bolted to a matchmaker, that would make removing things really quick. no need to remove grips then.
i reckon 20 minutes easily including tweeking in the gears. i've built a bike from a brand new frame in the car park near 18bikes in about 45 minutes, so if all your cables and everything are the right length it will be easy.
you might have two seat post diameters, so get another saddle to save having to swap them over.