You can use BAT on the remote site server (existing) and export the users to a csv file. Then you can use BAT on the new servers to import the same. To avoid confusion, i may suggest using a separate partition CSS, Device pool etc for the migrated phones, so that it will be easy for you remove them later. BAT versions differ from one version of Callmanager to another, so you may have to reformat the exported BAT file to the new server's BAT version format prior to import.
HTH
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Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus