If the serverA OS is compatible with SERVERB and you are only migrating 
some clients and not all ( if migrating all wny not do mmrecov & host 
transfer).
What might work is that you crete Client identicle ( except client id) to 
new server, you may be just be able to copy client index directory to new 
server before you create new client index and use this index for past 
recoveries, or else use nsrck -L7 client name to recover his index on new 
server( assuming your media is readable on new server)
Or you may  be able to use other switches withen nsrck to recover this 
clinent's index.
Last options:
Dont worry about recovering client index from past, just scan in the tapes 
as needed for recovery. ( assuming old server is around to either recover 
data or lookup media\saveset info) and it also depends on how far back you 
like to recover too.

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