How do I switch an 8.25 Small Bolt to a Big Bolt pattern?

So if I change my axels essentially what I’m doing is going from 5 on 4" bolt pattern to larger 5 on 4-1/2" pattern. This will open up a wider selection of wheels to me, correct? Can any one estimate how much it will cost to machine my drum Brakes and how much it will cost to convert to disc brakes in the back? I already have the front disc brakes. Also the 4" and the 4-1/2" is the technical measurement from where to where on the wheel?