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.
We have the same task as PRY except that we don’t have TN and Modeler.
Since we don’t have a license for a sandbox I would have to make upgrade and succeeding test on our development server.
Could you give me any recommendations about needed backup activities in order I could easily restore version 6.1 if our interfaces stop working with version 6.5?