Altcoins have a volatile price, and stablecoin follow the price of the dollar or gold. You can see one difference, and I haven't found a project that can convert altcoin to stabilcoin.If there is, please provide a link maybe I was left out of information.
Basically, the thing that make it different just how stable coin has commodity that used to backed it while the altcoin that is very volatile pegged to the fundamental of development progress.
When there's a coin called A and it wanna changed into the B that backed by gold or something else and coin A must think about how to eliminate the volatility through doing same formula like coin B.
It's quite simple. Anything is possible right now.