That's a good idea. There's many people who just don't need to spend the extra money on a MBP when they're not gonna be making good use of its features. It's never too late to admit you could've made do with a lower spec machine.
The MacBook is plenty powerful for most people and obviously more portable (you've got a 15" MBP right?). The only thing that might take getting used to is the lower resolution, especially if you were used to having multiple windows on screen.
Also, if you're gonna be getting the used version of the Blackbook then you should end up with a nice little cash surplus from the sale of your MBP.