I'm not sure exactly what you want to do but I will say I'm getting the impression that DM doesn't quite work as well as good old RM did. Especially when it comes to changing multiple files at once.
Select the target stuff in DM. Choose "Rename" command. Then use the "Replace" command (the replace text one -- over near find, not the replace document one). And key in the before and after text. I believe the need to choose the "Rename" command is new in ST9, because there didn't used to be explicit Rename and Move commands -- they were kind of "implied" by the Replace Text command.