727070
If you are already on a descrete code stay on that code and notify ATC immediately of your problem.
The only time that I could see squawking 7700 when you are on a discrete code is if the controller is extremely busy and you are concerned that you wouldn't be able to break in to announce your emergency, or the situation is of such a dire emergency (rapid decompression, etc) and you are making an emergency descent and you may or may not have time to notify ATC (fly the airplane first).
Regardless what you do, you data tag will stay associated with your target, the difference is that 7700 will set off an alarm and your data tag will flash "EM".
Mike
NATCA FWA