You do not need to buy a second 1000 user license. Your assumptions are correct. If you want to get a really good understanding of how the Juniper SSL licensing scheme works now - go over www.kevpeterson.com -he is the Juniper SSL Product Manager. Hit the category called licensing and you can read all about the new (now not so new) licensing and the impact on user licenses when box failures occur if you have an "unbalanced" scheme like 1000 - 500.
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