If you want to do it yourself a make a project out of it then more power to you, you'll like the gun even more afterwards.
If you're going to pay someone to do the work, then you ought to just buy another gun or barrel for the intended purpose. Chances are that gun shoots fairly well with shot loads as is, and you could get a seperate slug barrel for it for around $100 IIRC. NEF's are interchangeable, and the receiver is the only serial numbered part. You can get another barrel by mail order. It may have to be fitted, but IIRC that is only the rifle barrels. Either way it can be done through the mail.
Another option is you could get a mossberg 500, pump action gun for a couple hundred bucks. I bought one set up as a 12 gauge slug gun for $199 last summer. Very accurate and reliable, and made in the USA to boot. They have combo deals available with slug and smoothbore barrels both included as well for a few bucks more.