In updating my Metro app to Windows 8 RC, I only ran into one incompatibility - Microsoft revved WinJS (as they should!) The error I got was:
Error 1 Could not find SDK "Microsoft.WinJS, Version=0.6". C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets 1682 6 ToDoToday
All I had to do was delete the old reference from the References folder, right click and Add Reference, and select the new version. Looks like this:
Then I needed to change my references. The original default.html has this WinJS Reference code:
<link href="//Microsoft.WinJS.0.6/css/ui-light.css" rel="stylesheet">
<script src="//Microsoft.WinJS.0.6/js/base.js"></script>
<script src="//Microsoft.WinJS.0.6/js/ui.js"></script>
And I changed it to this:
<link rel="stylesheet" href="//Microsoft.WinJS.1.0.RC/css/ui-light.css" />
<script src="//Microsoft.WinJS.1.0.RC/js/base.js"></script>
<script src="//Microsoft.WinJS.1.0.RC/js/ui.js"></script>
And then everything worked!