Can’t install Exchange 2010 RTM when Exchange 2007 SP3 exists

I am faced with a very funny issue at a client where I'm migrating from Lotus Domino to Exchange.

As you might know Microsoft didn't release migration tools from Lotus to Exchange 2010 directly so the path was Domino -> Exchange 2007 and then Exchange 2007 -> Exchange 2010.

Since Exchange 2007 SP3 has been released a couple of days ago I've installed it on, what I was planning to be, a temporary server.

I was surprised today that I was not able to install Exchange 2010 and setup.com /PrepareSchema failed with the following error.