Create the mp3 files with the correct ID3 tags and make sure they have the exact same filenames as the files you've already uploaded. Log into your Blubrry Podcaster Dashboard. Click the Upload New Media Files link inside the Podcast Hosting box. From there, you can upload the replacement files and since they'll have the same names as files already on the server, they'll just overwrite the old files.
Doing this won't affect your stats, and it won't count against your monthly upload quota. The most important thing is that the replacement filenames must match the old filenames exactly. Also, the replacement files won't be available right away. It may take up to 24 hours before the new versions are available to download, but the old versions will still be there. This is due to how Blubrry's CDN handles caching of files.