I ran into a similar unfortunate situation upgrading from a 780m to an Aftermarket Eurocom 980m, I forgot to turn optimus on and was stuck with the 7 beeps boot loop ( Alienware Computer Does Not Turn On or Go Into Windows | Dell US ) messing with the CMOS battery did nothing for me, what worked for me then was swapping back to the stock GPU letting the system boot and reset the BIOS to its detaults then enabling optimus from Windows. Re-installed the 980m this time with optimus enabled and everything work like a charm this time. I read somewhere that you need to find a way to create an error on the POST to get it to reset back to its original settings I would suggest to keep trying replacing the stock GPU back and forth.