If you installed SQL Server 2012 RC0, your evaluation may have expired, or may be doing so shortly. So you’ll likely be wanting to upgrade to RTM- and you might run into a few problems along the way. That was certainly our experience, and when you’ve got SharePoint on the same box (as a demo BI machine, for example) you will want to avoid having to reinstall everything from scratch. Unfortunately there are quite a few problems along the way and you may be discouraged from persisting with this.
I thought I’d share my experience: in the end I managed to upgrade in place, saving quite a bit of time, and although many of the answers are out there, I’m putting them in one place for convenience.
Step 1: Turn back time
First off, you will be blocked from firing up SQL Server Management Studio, with a message about expiry. In the meantime, if your server has been running without a reboot for a while it could well be that your SharePoint BI is still working happily. If so, take the opportunity to download and backup any important reports and cubes you really can’t lose. Once the server is rebooted, the SQL Server services will not be able to start again, and the SharePoint functionality will be lost. You may well be scratching your head at this point, and considering a full reinstall.
Here’s the first answer- simply turn the system clock back a couple of months and manually fire up the SQL Server services. It sounds like it wouldn’t work- but it does- you should find everything back up and running! If you didn’t make those all-important backups, now’s the time.
Step 2: Upgrade in place
Next, you should be aiming to upgrade in place. In my case we were installing Developer Edition. There were two instances on the box- one for PowerPivot for SharePoint, and the other for ‘the rest of the SQL platform’.
You need to choose the option that says ‘Upgrade from SQL Server 2005, SQL Server 2008 or SQL Server 2008 R2′. It doesn’t mention upgrading from SQL Server 2012 RC0 but it does the trick.
I first upgraded the PowerPivot instance, then the other instance. Importantly, make sure the SQL Server services are running before you try the upgrade (you need the time travel trick for this), otherwise you’ll get errors.
The next problem you may get is an error saying Access to the path ‘C:\Program Files\Microsoft SQL Server\110\License Terms\SQLServer2012_License_DEV_1033.rtf’ is denied.’

Simply make the file writeable
At this stage I found the file, unchecked its ‘read only’ flag, and clicked ‘Retry’, after which the upgrade continued to completion.
I then upgraded the other instance, and fired up SharePoint. Nothing worked- PowerView reports hung, PowerPivot reports were blank or worse. All seemed lost. So that you don’t fall into the same pit of despair as I did, there’s just a few more things to do…
Step 3: Reboot, upgrade, and re-run
The first thing to do is a reboot. You should find all your SQL Server services are running, which is reassuring at least.
Next up, go into SharePoint Central Admin, select the SSRS service app, go into its properties page and press OK. This triggers an update of SSRS that brings it up to speed.
Next, get back into the SQL Server 2012 installer, and re-run the PowerPivot for SharePoint configuration tool. This detects you’ve already got it installed and you need to update it- follow the default options, and quite a while later it will be finished.
Finally, give it another reboot for luck. Your mileage may vary, but for me this was enough to bring the MS Bi in SharePoint back to life.