Support forum for Nagios Core, Nagios Plugins, NCPA, NRPE, NSCA, NDOUtils and more. Engage with the community of users including those using the open source solutions.
To answer the other part of your question, Upgrading from Core to XI is typically a straightforward process, but it can have some hidden gotchas. Not to be too commercial here, but that's one of the things that our company specializes in.