When I started looking into the upgrade path for this, I saw a couple of notes online about the fact that it was not possible. I turns out there is a Connect item on this issue (https://connect.microsoft.com/SQLServer/feedback/details/709371/no-option-to-upgrade-from-sql-denali-ctp3-to-sql-2012-rc0-version#details). In that item was a work around to use the SQL Server 2008 R2 upgrade option. When you start the RC0 installer, choose the Upgrade option which is last I the list as shown below.
This will launch the setup wizard and start the Setup Support Rules check after which you will be prompted to select the instance you wish to upgrade. In my case I have two named instances with CTP3 – DENALI and TABULAR as well as the Shared Components including SSIS. (NOTE: The CTP3 version number is 18.104.22.1680.19.) I started with upgrading my DENALI instance which had all of the services installed. On this instance, the Analysis Services instance was installed to support multidimensional databases. (The TABULAR instance only has a tabular Analysis Services Instance.)
As you can see in the Select Features dialog you are not able to change the selected features when upgrading to RC0 from CTP3.
My first “gotcha” – this may negatively affect my SharePoint 2010 install. In particular, integrated Reporting Services. I chose to risk it and continue.
I left the Instance name and accepted the supplied Instance ID in the next step. I made no changes on the next three steps – Disk Space Requirements, Server Configuration, and Error Reporting.
Second issue I ran into was related to Visual Studio 2010. I order to pass the next step I needed to update it to Service Pack 1.
The installer for Visual Studio 2010 SP1 can be found here: http://www.microsoft.com/download/en/details.aspx?id=23691. After I had that installed, I had to reboot and then continue with the SQL Server 2012 install. This allowed me to successfully pass the Upgrade Rules operation and I was now ready to upgrade.
After getting no errors during the upgrade, I was required to reboot. Now to check the instance.
Looking good. I was able to work with the updated versions of SQL Server 2012 RC0 in SharePoint as well.
All in all, it appears that the upgrade succeeded successfully as noted in the comments of the Connect item. I hope you have a similar experience.
UPDATE: I did run into an issue when trying to execute 2012 SSIS packages from Management Studio. A error regarding logging was raised. This issue has been posted on Connect and you can find the information here: http://bit.ly/vTUjcr. I have not tried the work around yet which requires fulling uninstalling CTP3 then reinstalling.