I am currently running Sram PG1070 12-28, but would like to upgrade to 11-28. I can get 11T cog for pretty cheap, but I saw I might also have to get a different lock ring then. Is that true, or could I use the one I am currently using? Thanks!
Yes, you need an 11T specific lock ring.
The outer diameter of the 11T lockring is definitely smaller, a 12T lockring won't let the chain sit properly on an 11T cog. The threaded section is the same size.