I have finally learned how to put on a sports bra without first taking off my regular one - a vital skill in a communal dressing room, and one that means I do not need to remember to put on a sports bra before going to work if I'm going to Pa-kua right afterwards. It's surprisingly simple.
Put sports bra on over regular one. Slip the regular straps off. Pull band of regular bra below band of sports bra. Unhook or pull over head. Yes, I know - it means regular bra is too big. But, honestly, I don't need more than minimal support for normal activities, so my bras go over my head already hooked. I still go the occasional day without one at all, although gravity affects even A-cup girls in time and those days happen less and less often.