I got back into road biking by putting a set of lightweight Bontrager slicks on my MTB. That did make a difference. There are two types of slicks - lightweight, and heavy, NYC messenger strength slicks. Unless you ride on really nasty roads, get the light tires. Try to stay off of gravel - it won't kill the tires, but the bike will be a lot more slippery than it was with knobbies.
Be aware that the seating position on a MTB is all wrong for distance riding. It is set up for out of the saddle sprint and coast, not for remaining in one position for long periods of time. Doesn't mean you can't ride the MTB on the road, just that you will probably get sore after a couple of hours.
I ended up building up a new road bike - carbon frame, Campy Chorus group. Comparing it to the MTB with slicks is like comparing a Ferrari to a truck. So if the road bug bites you after you've been riding on slicks for a while, look into a good road bike.