From my experience, the way I do it is create a snapshot of your current image, then once its done you'll see it as an option when launching new instances. Simply launch it as a large instance at that point.
This is my approach if I do not want any downtime(i.e. production server) because this solution only takes a server offline only after the new one is up and running(I also use it to add new machines to my clusters by using this approach to only add new machines). If Downtime is acceptable then see Marcel Castilho's answer.