If you were to take 10 images on 50 speed film, for example, and then needed to use fast film, 800 speed for instance - could you unwind the partially used roll of 50 speed and then put it back in at a later point?

Just curious about this. I read that you could, but I can't figure out how you would know when to stop winding, so as not to wind the film entirely into the roll, thus rendering the whole thing useless.

I use a Nikon FE2.