You do not have to replace the sensor every time you work on your brakes. You only need to replace it if it tripped the warning light on the dash, ie it rubbed through the contact.

It is still good practice to replace it, since it goes hard from the heat over time.

If you brakes are squealing from being completely worn out, then they could be worn out on the other wheel, or other pad, due to sticking calipers, but not worn out where the sensor is. If that is the case you should be looking at overhauling calipers as well.

Also, if they are down to metal, you should reconsider your plan to reuse your rotors.

It is possible that there is just a rock stuck in there, since you call it scratching instead of a loud grinding noise.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue