Confused, one of my sons hated to change clothes also. What worked for him was to wear his gym clothes under his other clothes. That way all he had to do was take off the dickies and polo shirt so he could participate in PE. The trick became getting him to put the dickies and polo back on, but the teachers were more apt to forgive the out of uniform attire.
Some Public schools do as well here, I think it depends on the principal. Honestly, I like uniforms because I think its easier for the kids, and parents overall! But its good to have say two types of pants like jeans or khakis, and maybe school mascot shirts?