Having not ridden my KHS in nearly a year, I ended up with both tubes not holding air. It currently has 26x2.1 knobby tires, and it had Continental tubes with presta valves on both ends.

I had one spare Specialized tube that said on the box it would work with 1.5 to 2.2 tire widths. I put that one on the rear wheel. I went to the LBS and asked for a new tube for the front tire (had rim and tire in-hand). The guy gave me a presta tube that said 1.5 to 1.9 tires and assured me it would be fine. I went ahead and installed it and rode up and down the street and it seems okay for now.

Will the 1.9 tube be okay in a 2.1 tire? I suppose it doesn't matter that much as I'm about to switch to 1.3 road tires and tubes, but I was just wondering anyway.

Tubes expand to the tire size and hold air.

I am upgrading my mountain bike tire which is a 2.1. My previous tire was a 2.2. The inner tube, which is still good and would like to use, is 2.2-2.5.

Will a 2.2-2.5 tube work with a 2.1 tire?

As long as the bead seat diameter is the same, any tube will work. The slightly larger tube may be difficult to stuff into the tire. Just watch out for the tube folding up or getting caught under the bead. If you get it in there fine, then it'll work.

Tubes do not shriink to fit. You might be able to get by or you might cause a problem and get a flat. Is that worth saving the price of a new tube?

ftfy