you need to completely disassemble the stub axles and hub to fit the new splash guards. To be honest its worth doing all in one go. If you don't do it you will have to releave your current splash gaurds lip for the larger disc to fit.
My experience was that the stub axle bolts to the axle but to get at those bolts you need to remove the hub. To remove the hub you will need some form of puller.
The reason you are meant to replace the bearing is because when you pull the hub it can destroy the bearing its a very simple cage bearing that push fits into the hub and normally gets left on the stub axle.