When I was Plat, on the LAX - SFO route, many times the GA could come in after I've been seated and advise I've been upgraded. I've always declined, since it's not worth the effort to get up, move my bag, and make my way up front.

I'd appreciate the upgrade before boarding, not after. Not an issue unique to AA, happens to me on AS too, where I'll likewise decline, unless it's a tcon, of course.