how to add index in replication scenario

Dear list,

I need to add an index for a new attribute in an active-active replication scenario.

I know I need to run slapindex to create the index for existing entries after I changed the configuration file (yes, still on 2.4 with slapd.conf). But what is the correct procedure to update both servers? I'm a bit worried that the setup will become out-of-sync if I just update one server at a time. Would it be better to stop the service on both servers and re-index the databases at the same time before going online again?

Regards,

Uwe