You're probably better off doing it over again.
720x480 is not only much lower resolution, it's a 4x3 screen aspect ratio. 1920x1080 is not only HD but also a 16x9 screen aspect ratio. It's wider. To take full advantage of the wider screen, you'd end up repositioning the graphics in your comp anyway.
Consider it a good lesson learned in checking your comp settings before you begin work in earnest.
There's a script that comes with more recent versions of AE that will scale up your comps should you choose to use it. I've never had to use it, so I forget its name. But it won't automatically move things around to take advantage of the wider screen aspect ratio.
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Go to File>Scripts>Scale Composition.jsx
Select a composition, select new comp width, type in the number, run the script.
That's it. Easy as Pie....
Pick the width you want. If after scaling the composition you need a different height to make the comp standard size just type in the new setting in comp settings. All of your layers, all of you animations, all of everything will be scaled properly.
Hope this helps.
Thanks for the input The scaling script is real nice. Thanks for the help !