Our old toilet sat 10 inches from the wall. The new one sits 12 inches from the wall. There is a crawl space beneath the house. How do I convert the flange?

How do I convert the flange? You do have options here.
1- Go in the crawl space and extend the closet bend and flange out 2". (best) **or**
2-. replace the flange with a 2" offset flange. Good luck, Tom

If you use an offset flange you may able to do most of the work from upstairs... ;)

PS: Most offset flanges only allow 1.5" offset, but that will work fine for a 12" rough in toilet (leaves you at 11.5" and that is perfect for 12" rough).

Mark