Yes, you need to delete the replication pair and then setup the storage unit. Then add it to Veeam and rescan it.

define use? as the replication target would be read-only, but it should be best to have it fully available to the backup tool in question? Your use-case is not clearly stated?

KB https://www.dell.com/support/kbdoc/en-us/000009425?lang=en is a rather old KB article about veeam 9.5, but still the same applies. It states that a replicated mtree target would be read-only, hence the perform a fastclone to another mtree on the target DD of that replicated mtree, to be used by veeam for DR as stated in the referenced document:

https://supportkb.dell.com/attachment/ka02R000000hA3YQAU/Veeam9.5_DDBoost_and_DR_csp_en_US_1.pdf

# filesys fastcopy ?
  filesys fastcopy [retention-lock] source <src> destination <dest>
                                       Fastcopy from source to destination

And then turn that fastcopy into a ddboost storage unit with "ddboost storage-unit modify veeam-repl-fastcopy user boostveeam", where in the example from above doc veeam-repl-fastcopy is the fastcopied mtree on the target DD and boostveeam the ddboost user to be used by veeam.