When I was in college I did the opposite. I would train at sea level in Louisiana during the fall and during the winter I would go back to Mexico City for the winter and train there at 7000+ feet. I can tell you that I would take me about 5 days to get used to the thin air. My main feeling was a little bit light headed and the sensation of not being to pull enough air when breathing so I had to ease into into and after one week I was acclimated and able to continue my regular training.

When I would come back for the Spring baseball season I could tell that high altitude training was very beneficial. I had greater lounge capacity.

My point here is to show you that at least for me it took me few days to be able to train at the same intensity that I was used to. How many days will you be in CO before the tri? I do not know that is there anything that can be done to expedite getting acclimated to the higher altitude.

Let us know how your tri went and your experience.

Good Luck