L.N.
You're not going to confuse her. She obviously 'gets it' and will understand perfectly that the pullup is just for sleeping in case her body doesn't wake her up to go potty. My almost 4 year old has gone thru 2 stages where he started wetting at night (growth spurts we think) and each time we put him in a pullup for a few weeks and not once has he ever questioned it or had regression from it. I say if she wants panties, go for it. If she understands it all with pullups, she'll understand it with panties even better, especailly if she gets to pick out her favorites! I think we worry a little to much about going back and forth during transition stages, but I think we are just teaching our kids that life is not black or white, one way or another....sometimes there are maybes, what-ifs, and in-betweens...and the gray areas don't have to be confusing. Good luck and congrats to your little one!
~L.