You said you wanted to maintain the best quality. 1440×1080 is higher quality than 720×480, more pixels. I suggested editing in HD because you said most of your stuff is HD. If your sequence is 720×480 that means you’re editing in SD.
In your position right now, the easiest and least technical option would be to edit in SD. Leave your sequence as 720×480. When you export your HD footage should look fine.