changing ebook cover

Hi guys, I'm facing a problem trying to change the cover of an ebook I've purchased from amazon. I've downloaded my new cover, changed it in the metadata page and then I've merged the metadata into the file with the dedicated button.
The problem is that everytime I send to my kindle the ebook it appears the old cover. I suspect that it's "recognized" by amazon and that the one that's in the cloud overwrites the one I've loaded.
Am I right? Is there a way of avoiding that and be sure that the ebooks I send to the kindle are the ones I've managed through calibre? Thank you!