Will Time Machine adjust old backups to reflect new TM exclusions?

I finally figured out that the reason my Time Machine backup takes up 3 TB of my 5 TB external drive when I'm only using 250 GB of my hard disk space is that it's backing up my massive work Dropbox folder.

I just told TM to exclude Dropbox from backups, but how can I free up the space it's taking up from my past backups? Or when it backs up now, will it delete Dropbox as a change? I'm still on Big Sur.

This explains why I've been having to replace my 2 TB drives every six months; they stopped deleting old backups and were full.

Thanks!