Try running it in nozzle mode with the belt off for 30 minutes and see if that doesn't help.  As already stated, the brushes may not be well seated.  Kirby motor brushes already are well shaped and shouldn't take much to seat them properly.  Did you use a seating stone?  That will do it quickly. 

DO NOT run it on high until the brushes are seated properly.  Also, check to make sure you wired the speed switch correctly. 

It sounds like you fried the armature when you accidentally switched wire.  I'd pull it and look at the brass segments on the commutator.  If they are melted, then it definitely is toast.  If you don't detect any melting, the next step is to test the motor field AND each segment on the commutator with an OHM meter.  I can walk you through that if you need help.  Don't run it anymore until you've done those checks or you'll also melt the insulation on the wires and something will catch on fire too.  If that happens, the smell is hard to get rid of.  With all that, it sounds like you're going to need a new motor field and armature.