You have to tell your games to use the new emulator.
Go to your Dreamcast platform, select all the games using Control + A on your keyboard, then press Control + E to open the edit tool, this will start the Bulk Edit wizard. In the first dropdown menu select what you want to change, in this case the Emulator, then in the 2nd dropdown menu chose your new emulator entry for Redream. Finish out the wizard and your games should all now use the Redream emulator.