Originally Posted by
Agfa001
But can I just purchase this single 12 cog in your link and swap (or have LBS swap) it with the 13 that is on there presently?
Yes.
An issue you might have getting just one new cog is mismatch in cog wear. If the old cogs are really worn, the chain might run fine on them but slip or jump on the new cog. And if you get a new chain, it will run fine on the new cog but it might slip on the old cogs.