New tires for race day

Unless you are really worried about your current tires, I'd wait.

X2

I did this once, but I made sure to ride for 30 minutes the night before.  Just to make sure everything was good.  Had no problems.  But I wouldn't recommend doing it if you don't have a chance to ride before the race.

If they ain't broke...

Just make sure the tire/tube is installed properly as noted above.  Otherwise, I don't think it's a major risk.

My next race is on Sunday and I just changed my tires. One of my Vittoria racers blew out this past weekend and I had to put on an old tire to go out for my ride. Upon inspecting the old tire, I just don't have confidence in it. So I picked up two Michelin Pro Race 2s yesterday because I don't want to race on a mismatched set. However I plan to ride on them twice (today and Saturday) before the race.

On a side note, man, those suckers are hard to get on! I feel like I should log that time as a workout.

If you change them make sure to ride an hour or so on them before the race. If you can't then I wouldn't do it unless your current tires are in really bad shape.

I put on a new rear tire and went for one short ride before the race.  No problem.

If your are replacing just to replace I would wait.  If the old ones are broke then replace them but be sure to get a short ride in first.  You never know if they are on right or not until you ride on them.

When tire come from the factory, they have a slick surface, almost like an oil. You would like to get this off before your race. I have read, and a lot of roadies that I work with try to get about 100 miles before their "A" race. They will usually use their race wheels/tires for races leading up to the race and will usually be a little over 100 miles. This wears the rubber a bit and gives you a good grip of the road.