Pictures and videos don't get better. They can get prettier, more colorful, and so on, by cleverly removing "bad" information about wiggly lines and making them straighter, for instance, but changing the format cannot ever add more information. At best, changing pictures removes information from them.
So if I change an old 240x360 pixel picture into a 1200x1800 pixel image, it will have "higher resolution" with 25 times as many pixels; it'll take up more space; it'll make a bigger print; but it'll be just as fuzzy as the original. (But you can stand farther away from it!) All that happens is you make 25 copies of every pixel, so effectively the pixels are fatter. To change from 720 to 1080, you just pack in pixels that look like the ones around it. It doesn't look better. Any apparent increase is essentially fake.
But if you're going to view a 720 video and a 1080 video on the same screen, then the software that plays the videos will already be adding pixels to both of them to make them fill the screen-- it just adds more made up pixels to the smaller image. If you change the 720 to 1080 during the export, or if you let the player do it automatically when you watch it, you get the same picture.
So I wouldn't worry about it.