Assuming you at one point upgraded to XNA4, it's not that hard. About 3 years ago I decided to see how hard it would be to get a game I released on Xbox Live In

Assuming you at one point upgraded to XNA4, it's not that hard. About 3 years ago I decided to see how hard it would be to get a game I released on Xbox Live Indie Games running on Monogame, and I had it building and showing the main menu in about 2 evenings of work, and worked out most of things that needed to be changed (namely storage and some Xbox Gamerprofile stuff, I believe) after about a week. Had I known it would be that easy I would have done it years ago. Actually lead to me remaking the game in 3D and turning it into a proper sequel (which I'm still working on).