How to adapt new pump to old casing

Wanted to save $$$ by trying to DIY installing new shallow well jet pump.
Bought a Wayne pump only to discover my old pump had a built-in flange that bolted onto the 3-inch well casing. What do I do now?

If that well head your talking about is a casing adaptor, you can't just remove it. There is droppipe hanging from it with a jet on the end. Once moved, you must remove it and replace the leathers then reinstall it. This is not as easy to do as it was for me to type it either.

Casing adaptors look like this: