In the web.config you can put parts of the config in separate files. This is useful when updating a web.config without overwriting installation specific settings.

In web.config:

<appSettings configSource="appSettings.config">

In appSettings.config:

<?xml version="1.0"?>

<appSettings>
  <add key="PageInspector:ServerCodeMappingSupport" value="Disabled" />
  <add key="Telerik.Web.UI.StyleSheetFolders" value="~/Skins" />
  <add key="Telerik.EnableEmbeddedSkins" value="false" />
  <add key="Telerik.Skin" value="Kis3" />
  <add key="Telerik.ScriptManager.TelerikCdn" value="Enabled" />
  <add key="Telerik.StyleSheetManager.TelerikCdn" value="Disabled" />
 </appSettings>

But the upgrade wizard cannot handle this and modifies the web.config like this:

<appSettings configSource="appSettings.config">
  <add key="Telerik.ScriptManager.TelerikCdn" value="Disabled" />
  <add key="Telerik.StyleSheetManager.TelerikCdn" value="Disabled" /></appSettings>

Resulting in different errors.

It's easy te correct this, but it's a little annoying.. :-)