There's a little less tension on the strings. Other than that, no big deal - don't overthink it. Keys hit the transpose button for the entire gig, harmonica has to play a different harp, brass / sax plays down a half step. They're the only ones for which it really changes. Make sure the lower songs aren't to low for your singer. It's unlikely that your truss rod will need any tweaking, but check it out before you get to the gig / rehearsal. If you have perfect pitch, it may bother you initially.