I have a full Ultegra tri bike and a full Dura Ace Road bike. If I decide to use my road bike wheels for a ride on my Tri-Bike, do I need to switch cogsets, or will the Dura Ace Cogset work with Ultegra (and vice versa)?

I'm hoping to get some tubular race wheels asap, but there is a chance that I'll get the rear wheel first, leaving me with one tubular & one clincher; I may have to ride one race with that set-up. Of course I don't want to carry any more stuff than necessary and I've heard that if you get a flat in a race, you can put a tubular tire on a clincher rim to get by. Is this true? If so, is it a safe solution?

thanks for your help.

Barry K.