Changing during boarding generally does not impact lav use. I don't recall ever waiting for a free lav during boarding, as people are concerned more with getting settled (and rightly so) ... I generally board early to give me time to change.

Changing back is more challenging. I try to do it before the meal carts come out, and definitely avoid the last-minute rush prior to landing. I have been known (once) to wear my non-couture PJs into the terminal, and changed there.