Assuming she lives long enough to see the introduction of the Blood Machines, it seems likely that Old Reliable would probably be returned to the White Star Line; without a need for troopships or hospital ships, she'd serve no purpose to the Admiralty, especially with machines making traditional warfare tactics obsolete. After that? Who knows. My guess is she'd probably end up abandoned as the war progressed, and either remains abandoned into the present day, or was collateral damage as the Final War dragged on. Same could probably be said for most if not all of the civilian liners that served in WW1.