ECU flashes Kevin upgrade the firmware held by an
EPROM chip within the ECU by overwriting the code it previously held. Since the EPROM represents non-volatile memory the new firmware code is permanently locked (burnt) into it, until perhaps another firmware update/flash that is. Much the same on your PC, smart phone and many other electronic devices nowadays where upgrades may be offered to correct previous programming errors or add new features.
What will be lost by a battery disconnection is the adaptive mapping for the engine, held in RAM within the ECU, and continually updated to tune your particular engine. It will simply adopt the base map in the firmware on battery re-connection with adaptive mapping re-learnt in short order. No need to mess around with temporary power sources therefore. Note some modern cars may require a charge profile reset after fitting a new battery but not your Morgan
