In the middle of a ride? No, I have never.
If I'm going on a ride where I think I'll be worried about keeping pace, I'll either make sure I'm geared right when I leave, or I won't ride fixed.
Suppose that could be true for some. I wanted to see if I could handle on a lower gearing, turns out I can't, luckily I had a flip flop hub, which I knew going into it. Regardless of either fact I wasn't having trouble keeping pace with the ride, so it wasn't that severe of an issue. I just couldn't keep pace with the pack at the front which frustrated me.
I just figured since I'm running a flip flop hub, and I have the option to change gears if I want. Why not take advantage of being able to try things out.