After the BUILD conference we now have more information on how Microsoft plans to do the upgrade from .NET 4.0 SP1 to .NET 4.5 and what this upgrade will imply.
Contrary to all other upgrades in the last years this will not be a side-by side upgrade but instead an in-place update. The .NET 4.0 SP1 framework will be replaced on the system during the upgrade process. Furthermore it seems that you won’t be able to go back once you have done the upgrade.
To allow such an update Microsoft will have to assure that old code that ran under .NET 4.0 SP1 will still run correctly under .NET 4.5 which is not an easy task. But the Microsoft staff is currently working hard to assure everything will work seamlessly when the final version of .NET 4.5 is shipping.
I will post a migration tutorial when the final version will be published and try to help you in having a good migration strategy.