No. You can't change cluster size after you've formatted the drive. But, again, I mentioned that disclaimer. It won't affect you. You want the optimal settings for plex, not the theoretical minimum and maximum for the drive. The only salient question for you is "will a larger cluster size negatively impact plex's ability to serve numerous large video files on the fly, or the ability of other applications to manage a media library?" And the answer is no. It may have some theoretical inefficiencies for some purposes, but you don't care about those. They won't affect your use case in the least.