How to convert your dedicated server from RtM to RtM: Durin's Folk.
The config file will have this section in it.
; Comma separated list of DLCs to apply when upgrading worlds without them.
; By default this will always be empty as upgraded worlds cannot go back.
; Most commonly used when loading old worlds after new DLC content is released.
; If you wish to upgrade your existing worlds to new DLC content please fill this in with OptionalDLC values above.
UpgradeOptionalDLC.Array=""
If you leave it blank, then your save won't be updated. What you would need to do, is change it to this.
; Comma separated list of DLCs to apply when upgrading worlds without them.
; By default this will always be empty as upgraded worlds cannot go back.
; Most commonly used when loading old worlds after new DLC content is released.
; If you wish to upgrade your existing worlds to new DLC content please fill this in with OptionalDLC values above.
UpgradeOptionalDLC.Array="DurinsFolk"
If a player has the Durin's Folk DLC, they can join the server. If they don't, they will be stopped at the code screen. Characters that have the DLC can still go to worlds that aren't DLC-enabled.
Once your world is updated to the DLC, it CANNOT go back. Make a backup before you do it.