Up until recently there was no easy way to move ID server functionality between repservers (eg, current ID server is to be decommissioned).
NOTE: While there is a manual method (direct updates of RSSD tables; updating repserver *cfg files) for moving ID server functionality between repservers, said method is not outlined in SAP/Sybase Repserver materials.
With RS 15.7.1 SP300 a new command (sysadmin idserver) has been introduced that provides an official (read: *supported*) means of migrating ID server functionality between repservers.
The attached document provides a brief overview of this new functionality along with some examples demonstrating the migration of ID server functionality between repservers.
NOTE: scn.sap.com does not appear to allow uploading of *.doc or *.pdf files; I’ve converted the file to *.htm format and while not an exact representation of the original *.doc file, it should be readable in most (all?) browsers … *knock on wood*
August 2nd, 2016: updated the attached document to include a bullet point about a potential downside to having a backup ID server: if the backup ID server is down then the primary ID server will fail to issue new IDs (eg, create connection)