If the deduplication setting are changed for an existing job, when will the new setting...

https://helpcenter.veeam.com/archive/backup/95/vsphere/compression_deduplication.html

To apply new deduplication settings in backup jobs, you must create an active full backup after you change deduplication settings. Veeam Backup & Replication will use the new block size for the active full backup and subsequent backup files in the backup chain.