lol... why not just export 720? 1080 will be downrezed a little, making everything the same. If you go 720 to 1080 you are stretching the 720 footage. Whats your delivery medium?
This project was shot using three different cameras, the HPX170, the EX1 and finally the 7D (everything after about the 3:00 mark when we go to Brazil).
The FCP timeline was set up as a 1920x1080 23.98fps ProRes project. The HPX footage was all shot 720 and dropped in the timeline, the EX1 footage was 1080 and the Canon 7D footage was a mixture of 1080/24p and 720/60p. The 720/60p footage was, for the most part, re-timed to 24p in Cinema Tools for slow-motion effect, though there are some straight-up 60p shots that were just dropped in the timeline.
As always, it's Vimeo, so I wouldn't go to far in judging the sharpness and all that. I merely post this to give you an idea of what the finished product can look like with different cameras, different frame sizes, etc.…