I would suggest installing as a parallel install. That would give you the capability to still stay on 10.5 whilst you get comfortable with version 11.1. You can migrate all of your documents, configurations, thresholds, analyst rules, and database configurations to the newer version, test, then turn off the older version and start using the newer version.
To add to Jack's response a parallel install is only available on non-Windows platforms on the same box. A parallel install for Windows is not possible without having a new server. You have to do an upgrade or have a new machine to perform the install. Prognosis can typically 'work down' though but there is a limit.. such as 11.x cannot interact with any version of Prognosis below 10.2 at all.
There are definitely cases though where Parallel installation make not make sense or is not possible due to system resources or limited access upon those resources, or additional supporting infrastructure configuration required even on a non-Windows platform.
I like to think of it as a 'clean' install vs. a parallel install in many cases, but there are definitely risks with the upgrade and the parallel strategies especially when you take multiple versions of Prognosis into account and your deployment architecture. A clean / parallel install will involve many more manual steps one has to perform. An upgrade may fail and you may be forced into a clean upgrade anyway.
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Christopher R Souser - Payments and Infrastructure Consultant – MSci. PA, CISSP, ITIL.