Your NIE numbers should remain the same.
There are - of course - no 'Residencias' any more. Unfortunately, even if you had gained permanant resident status during your earlier stay, this has now been lost by virtue of being absent from the country for over two years. Hence, you will in this case need to apply for a Certificate of Registration for a Community Citizen. This is not the same as a 'Residencia' but is required if you remain here for more than 90 days at a time. You will have to satisfy the income and health care coverage requirements now being insisted upon.
For further guidance see:
EU GUIDE TO FREE MOVEMENT
British Embassy Translation of Spanish Law