RE: 870 Barrel Switching
Yep, I always have to zero after taking on and off with either a 1187 I had or my 870, or my gold deer hunter. It shouldn't be off by much. You just might have to lower 2MOA or something like that.
Even after cleaning, it changes zero some.
Even rifles, if you unstock them, and put them back together can change zero. Especially without good bedding.