I recently bought some sexy shrimp from liveaquaria for my breeding project. You never know exactly what you're gonna get, so I ended up with 3 small males that were less viable for breeding than a couple others, and I put them in the display tank instead of the breeder.
The last time I visited my lfs, looking for sexies, he mentioned that a lot of shrimp can change sex when needed. I had never witnessed anything like that with sexies before, so while I assumed it was likely true, I figured they must have a fairly early point-of-no-return.
Well over the last few weeks, I've watched as one of the small males in the display has rediscovered herself. The transformation is complete and I expect to see her pregnant any day now.