Lots of incomplete, incorrect, or misleading information here.
Progressive modes are at 60 FRAMES per second, not fields, and 1080i is actually 1080 lines of resolution - more than 720p, but at 60 fields per second. This is very important information.

A picture really IS upconverted frpm 720p to 1080i, however at a slower total framerate. Listing 1080i as "1920x540" is just plain wrong, mathematically you can account for the total pixel bandwidth this way, but it is still wrong. Imagine if I called a 10'x10' square a 20'x5' square because it has the same area. Upconvert and Downconvert actually do mean something when you understand what is really going on.

Running a game at 720p on a 1080i native resolution set will not cause problems as long as the image scaler isn't crap.