Hello,
It all depends on what your current backup and recover policies are, if 
you retain the tapes offsite for longer then a month, you can perhaps keep 
the old server for month or two, meanwhile, you can test recovering data 
backed up via your HP-UX server from Windows, if you are satisfied then 
you can retire the old server, if not the other option would be to recover 
old data , each client at a time to a alternate location for your extended 
retained tapes and back them up using windows server again. ( Test recover 
one more time). Also keep the original tapes as failover just in case 
scenario until their retention period expires.
In either case, explain all  the possibilities to your boss and get him 
onboard with the plan so that it's not a issue later.
I don't have experience doing such a conversion so may be other panel 
member can share their experience, however, from what i know, networker 
writes in open tape format and so data backed up via Solaris is 
recoverable via windows server, yes using scanner etc.
You may also be able to use mmrecov to build new windows server, 
preserving all your  networker  database.
Why not just do a Test of such an recovery  in the lab first ?

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