The upgarde of main components to 6.5 will not be a problem as long as your implementation was created using best practices in development.
IS, Broker, TN Sever, TN COnsole, Developer, Modeler (6.5) will be ok to upgarde. The other components you mentioned are more difficult.
Workflow and Manager are not GA yet and only available as First Customer Ship releases. One more factor is you will need to use MyWebmethods (Portal) to use Monitor and work with your models and Manager Console is now inside MyWebMethods. So there will be some extra components to install and configure and learning curve to deal with MyWebmethods Portal.
#webMethods-General#webMethods#webMethods-Architecture#Integration-Server-and-ESB