http://m.rei.com/product/777295/humangear-capcap-water-bottle-top

I would recommend the above product to convert a wide mouth bottle to a narrow mouth.

The narrow mouth makes it much easier to drink from on the run in my opinion. A wide mouth is easier to clean and it can be used for food as well as drink.

The converter let's you have both bottles in one. The strap broke on my first one but I carabiner clipped the strap to my bag every day for many months and it got knocked around. I tied accessory cord around the lip of my Nalgene and clipped from there and so no issues with the second one.