I would suggest that you rebuild your servers rather then upgrade. Reason being there are alot of tools that you may want to employ such as SMS(used to push client upgrades) that do not run properly or in some instances at all on an upgraded server. From your comments stating that your hierarchy is fairly uncomplicated it would make it less work. However if your not going to ever use these tools an upgrade is pretty straight forward. I will say its very dangerous to say that in the future you wont need these tools, I’m sure you understand the importance of having a scalable infastructure. Hope this is helpful in your process of making a decision.