I would just go in more detail there, its simple to install the new template for it but the problem lies in the module positions/names. So i what i would do is plan ahead.
First get list of your module positions and module names, then check the layout of the mero and plan where to put those existing modules. The T3 framework is missing one thing right now to make it perfect framework and that is the module position naming issue i have with it, it is not unified, so switching templates don’t work without a lot of work in the back-end. Best practice would be to have standard names for module positions for all templates OR to have the actual backend where you can rename module positions to match your previous template.
Right now you can assign the position names to the actual layout placement areas BUT YOU CAN NOT rename them. You are given popup window with available names and positions.
So to make it painless what you have to do is what i described at start and everything else should work fine.