My chicken has been sitting in the buttermilk since last night. Can I let it brine until tomorrow evening? Change of plans and I won't be able to...

what HalfPint said. 48 hours or so is too long to marinate anything other than Sauerbraten. The meat will lose any semblance of texture.

I would not brine chicken any longer than 24 hours or the meat will turn to mush. If you can't cook the chicken tonight, I would take it out of the buttermilk, wipe off any excess, and keep your chicken refrigerated until tomorrow evening when you can cook it.