Oh, I did the exact same thing when starting to ride clipless, except I wasn't trying to switch feet, just unclipped the left foot (like I'd practiced and practiced and PRACTICED that entire stretch), slowed to a stop at a pedestrian crossing and promptly toppled over onto my right side.

But being able to switch feet can be handy I've found out - I just snapped a part on my left pedal which means I can only use the one side. Finding it is a hassle in traffic, so I tried to unclip my right instead for a few days. Couldn't do it. Like folding my arms the wrong way.