To migrate your DPM protected servers from one DPM parent server to another, follow the procedure below...
On the client machine (agent) to re-targeted.
1) Open a CMD promt, and CD to "C:\Program Files\Microsoft Data Protection Manager\DPM\bin"
2) Type "SetDpmServer.exe -dpmservername yourdpmservername"
On the new DPM server you wish to use
1) Open a powershell prompt, and run "attach-productionserver.ps1"
2) Answer the prompts!
See the screenies below!