Has the Special Tourist Visa replaced the Multiple Entry TV, and can it be converted to another type of visa in Thailand?

AND the STV visa cannot be changed into any other visa inside Thailand. Per the specific rules of the STV visa.

nope the 6 month METV is still available and the Special Tourist Visa was a totally different thing. On the eVisa site you select Tourism & Leisure and then multi-entry and that is the 6 month METV

Also keep in mind the Special Tourist visa program runs out in Sept, and anyone who gets one now and enters can only get the 90 day entry stamp not the two 90 day extensions that visa allows. Unless they renew the program it's over in Sept