I usually have my own clothes to sleep up and I change before boarding in the lounge. I'll put on my yoga pants and t-shirt and then cover with a hoodie. If I need to change on the plane, it's definitely in the bathroom. A lot of times on the wide-body planes, the bathrooms aren't that tiny and even if they are, I can manage. I would never, ever change in the open. It's an airplane with strangers, not my house.