satya,

Is a reset out of the question. It's easy to know if you have a cold reset or a warm one.

You can easily build a switch circuit that would reset in either external or internal mode.

You are aware that any fetches beyond the internal ROM address range fetches from external memory aren't you. Perhaps you can allocate a pin with a switch that tell the internal firmware to execute either internally or externally from the "extension" external ROM.

regards,
p