I am having the same issue. Claimed the amazon offer / attached it my battle.net account. But I can’t download SC remastered, only the regular version. When I try to “buy” it on the store it says Congratulations you already own it. I tried deleting the “battle.net” folders to get rid of the cache, restarted the computer and it’s still not updating.
Edit: nevermind, got it to work by toggling “Enable HD” under settings/video. It was already “on” with the old graphics, but toggling it on/off got the remastered version.
I’ve been messaging support for days trying to get this fixed and they just keep talking in circles, getting nothing done. I think the plan was to have it not work and then not fix it for people until the offer was over, so now they don’t have to give away anything. Pretty rotten if you ask me.