I did this at my last house. I removed enough 2 x 4's inside without taking off any siding so I could re-frame a door opening with a header. After I had it framed the size I wanted, I took a chain saw and cut open the siding to the inside of the 2x4's I had framed. Cleaned up my cut with a sawsall and knocked out the foundation by drilling a bunch of holes and then breaking it out with a big hammer. Cleaned up that with a grinder.
Installed the new door I bought with the Jam attached and screwed it in. Sealed it up and caulked all around and I was done.
When you get to looking at it, its really not that hard.
Mm