Lehigh most certainly still sells the conversion kit. They quit for awhile until the Knight contract ran out.

Plug with vent liner $16.50 https://sskfirearms.lehighdefense.com/all-products?product_id=259
Adapter for the Extreme bolt $14.50 https://sskfirearms.lehighdefense.com/all-products?product_id=258

Call Knight and ask for Steven. Its $61.25 for just the Extreme bolt housing. You dont need the entire bolt. Just swap the guts. Takes 2 minutes.