The scale of your plan depends largely on the number and type of services you're running on the server.
Some services transfer seamlessly. Others need minor tweaks. Some are a PITA.
The criticality of the server is also something to consider - if the server is business critical and can't be offline for extended periods of time, you might be better off getting another server, setting up parallel services on that server and migrating the users/services over, rather than trying a fork-lift upgrade on an active server. Sure, it adds to the cost, but at the end of the day you end up two two servers for backup/redundancy, if nothing else.