I believe it can take up to 72 hours for the change to be in effect. Sometimes the old cover will show up on the sales page, but the 'look inside' will show the new cover. I think if you download the free sample, it will show whatever cover is currently going out on any new sales.
I changed mine on Sunday, and it still isn't showing up. I think six days is a personal record for me. My previous record was five days. If it doesn't show up by tomorrow, I'm going to reload it. Good luck with yours!
I'm in the exact same situation. Updated my cover Tuesday, still not changed. Guess I'll just have to be patient, but it's maddening--the book's anniversary is tomorrow, and I was hoping to have the new cover up by then.
I also re-uploaded mine last night, and this morning, it's showing. Don't know if it's because of the re-upload, or whether finally, after seven days, the original upload showed up. That's the longest I've ever had to wait for a cover.