We are using xdt:Transform="Replace" which basically replaces the connection string of our development DB and it works perfectly. See below:
Development connection string (in your case web.Debug.config):
<connectionStrings>
<add name="MyDB" connectionString="Data Source=DebugSQLServer;Initial Catalog=MyDebugDB;Integrated Security=True" providerName="System.Data.SqlClient" />
</connectionStrings>
Production connection string (in your case web.Release.config):
<connectionStrings xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
<add name="MyDB" connectionString="Data Source=ReleaseSQLServer;Initial Catalog=MyReleaseDB;Integrated Security=True" xdt:Transform="Replace" xdt:Locator="Match(name)" />
</connectionStrings>