I think I know what you're asking, which is how to replace the old files with the new files in bulk. Unfortunately, there's no mechanism to directly replace a set of files with another set of files. You can only do one file at time with another file.
I've recently been upgrading my collection from MP3 to FLAC and I will tell you what I do. The "FLACs" here will be your higher bitrate MP3s.
1. I import the new FLAC files to the inbox
2. For each album, I right-click, search on the album. This brings up the new copy and the old copy. You need to do this from the Inbox or else you'll only search your library and find the old set. I make sure I'm in Tracks view sorted by file location and then album, so the two sets are distinct. The new set should also have an INBOX indicator.
3. I use the "Additional Tagging and Reporting Tools" plugin here. I right click the old set and copy the tags I want from them, which is date added, genre, play count, last played and occasion (which I use to flag files as "Explicit" when they are). Can be whatever library information is important to you. Actually, I created a keyboard shortcut for this because I was doing it so much. I wish you could copy playlist membership, but you can't.
4. I paste the copied tags to the new set. Also a shortcut.
5. I manually add the tracks from the new set to the playlists the old set's files were in. I have a playlist column in my display so it's easy to know.
6. I move the new set to their organized folder
7. I delete the old set
8. I add the new set to library