From personal experience, because I just went through this with my new snake, I would wait at least 24 hours before changing out the bedding, since you don't want to disrupt him while he's digesting, he will be okay for an extra day or so without the cypress. As far as misting the aspen goes, try not to mist it directly, I spritz the water around the cage but not straight onto the aspen just as a precaution. I have aspen on one side and cypress mulch on the other, I mist the cypress daily, as well as keep a damp towel over top of one half of the enclosure and it has really helped to boost the humidity.