If you read the PAD it is very clear why they are making this an AD. In July last year a helicopter crashed in the US and it had bladder tanks fitted, The aluminium shell was badly damaged but the bladders held out fine and there was no post crash fire,
IF you read the SB it explains all about the bladder tanks. You take the old tanks off and bin them and fit the new tanks that have bladders installed inside, Expect each aircraft to cost around £10,000 by the time you've bought the tanks had them fitted (40 hours labour) and then got the new tanks painted.