change storage unit to replication source

Hi All, 

we have 2 netbackup domains (N4, N8) with 1-1 DataDomain attached to it. 

I'd like to setup Netbackup AIR between these two domains. is it possible to change the currently used storage units mode to replication source? or do i have to create new storage units? 

On datadomain the ddboost assoc. is setup, but netbackup is not accepting the new SLP, saying the STU is not capable to replicate from.

sysadmin@ N4# ddboost association show all
Local Storage Unit   Direction      Remote Host                    Remote Storage Unit
------------------   ------------   ----------------------------   -------------------
mssql                replicate-to   N8                             mssql_N4
------------------   ------------   ----------------------------   -------------------



sysadmin@ N8# ddboost association show all                                                
Local Storage Unit   Direction        Remote Host                    Remote Storage Unit
------------------   --------------   ----------------------------   -------------------
mssql_N4             replicate-from   N4                             mssql
------------------   --------------   ----------------------------   -------------------

thanks 

Gergely