First choice: find a bathroom. I often start my rides near an outdoor shopping center with a decent public restroom, and there's usually a convenience store or fast food place near the start point for other rides.

Second choice: change in the front seat of my car. It's not the easiest thing to do, but it works. I have several reflective sun shades that I use to cover the windshield and side window in case anyone walks by while I'm indecent.

(I won't change in a porta-potty -- they're too dirty and I worry I would accidentally drop something.)