I flew to Denver 3 months post-op and didn't have any problem with the altitude. However, the flight delay I had out of Dallas-Fort Worth zapped my energy reserves.
If you've had lung problems or pulmonary embolisms, you might think twice about going to Denver. A friend in S. California had 2 PE's and 1 DVT about 2 years ago. Was hospitalized, put on warfarin. Several months later, maybe 6, she had to go to Denver. She told me that she began having breathing problems on the jetbridge. She managed to get through the weekend, but hasn't been back to Denver much since then.