Say I want to change an 18t to a 16t rear track cog. Who makes a tool to facillitate this?

https://sporting-goods.search.ebay.co...Z1QQsofocusZbs

chain whip, check the LBS

I have a chain whip, but what about the tool that grabs the track lockring in back?

Oh, well a bottom bracket lockring wrench will do it.

https://cgi.ebay.com/PARK-BOTTOM-BRAC...QQcmdZViewItem

i have the shimano tool. It does everything regarding the cog and lock ring. Only 1100 yen, about 9 bucks usd. I guess it would be about 15 in USA?

You might want to do some skidding after you get the lockring off before you go at it with the chain whip. it will make your life a little easier.

Rotatfix:
https://en.wikipedia.org/wiki/Rotafix

Not all chain whips fit 1/8 cogs.

For the lock ring you need something like this lockring wrench from Hozan:
https://www.hozan.co.jp/cycle_e/catal...edal/C-205.htm

or a Makino Pro Tool Super:
https://www.businesscycles.com/trtool_makino.htm

For the lockring I took a large flathead screwdriver placed it in a notch on the lock ring, and hit the back of the screwdriver with a 5 pound weight. The screwdriver was angled in the proper direction to loosen the lockring.

then for the cog I used one of these that I had lying around: https://www.specialized.net/ecommerce/shop/layout.asp?product%5Fid=030X389&12''-Utility-Plier-R212C

Make the opening in the wrench very big then grip the teeth of the cog and pull like you've never pulled before. This technique destroyed my knuckles (from scraping the spokes when the cog finally got loose) and it destroyed my cog (I was going to throw it away), but I didn't have to buy any specific bike tools.

+100

It's better than any chainwhip.

Also, not easy to find a 1/8" chainwhip. For example, I don't have one such.

As for the lockring, I have a Hozan lockring tool, and it's the sweetest thing. I recommend it.

just use vice grips.

You can also use a vice and a length of chain to remove the cog too, you get a lot more leverage turning the wheel...