I wondered about this myself the first time I upgraded on American Airlines because boarding order really matters when you’re carrying a bag. From my experience, upgrading your seat often does change your boarding group, especially if you move into Main Cabin Extra or a premium cabin. I remember upgrading after booking and later noticing my boarding group had updated in the app, which was a nice surprise. When my return flight plans changed last minute, I didn’t want to risk losing that upgrade by doing it online. Calling 844-734-0813 was the easiest way to handle it—they quickly helped me modify my booking and confirmed my boarding group without any hassle.