There are a few variants of networked Monarch - I may not be totally up to date, but there were Citrix, Concurrent and Named-User in my day. Of these, the first would be trivial - upgrade the server and automatically everyone else is up to date. The other two both have a component on the client PC, and I doubt there's much chance of you getting it to work across versions.
Bear in mind that different versions of Monarch require different .NET frameworks to be installed also - so check the minimum desktop requirements before upgrading. Datawatch might be kind enough to let you run two servers - so that you can gradually move users over from v9 to v10.
I beleive our install is concurrent where we have it installe don a physical serve rnand can install as many clients as we like but only a set number can access at a time.
I was thinking of the dual install and then upgrade the clients gradually.
Those anyone have any suggestions on how they upgraded something similiar?