Changing O2 Sensors

Hi I have a 2004 Holden Rodeo V6 3.5L Isuzu (Petrol).

The fuel economy has gone out the window so I'm doing plugs, air filter & O2 sensors.

Gone from 500kms a tank to around 350kms a tank.

Engine runs fine, no smoke doesn't use oil.just poor economy, 

My Question is when replacing the O2sensors do you just unscrew the old ones out and screw the new ones in and plug them in,

do you have to clear the ECU memory or will it do it it's self. 

Thanks in advance for any help.

Cheers