What is the proper way to decommission a solarwinds server, after building a new one.
Would depend on your requirements, but at a minimum you would want to park your license. Sounds like you have already done so, but just in case migration guide is found here: http://www.solarwinds.com/documentation/orion/docs/OrionServerMigration.pdf
Go into license management and deactivate the SW licenses, then uninstall SW or reformat or whatever.