You'll have to unscrew the tailcap module to replace the rubber boot, the squeeze it in form the back method won't work and you will compromise some water resistance. You'll need needle nose pliers or split ring pliers to take out the switch module, unscrew the module by putting the plier tips in the holes on the back of the module and unscrew, it should be easer form then on.
Check
this thread for pictures.
Well, mostly this picture(taken by StefanFS)
The tailcap disassembled. The reverse two stage clickie module has an 10 Ohm smd resistor for the low mode. The switch itself seem to come from the same manufacturer LumaPower uses. High quality in my opinion. Click on to low level, click again for high. Third click is off. The threads are not anodized so it's not possible to lock out the switch by unscrewing the tailcap a few turns. Why the switch module body has off center grooves cut into it is anyones guess. A hungover worker or somebodys idea of a joke maybe?
Edit: I do not have a A8 so I'm just repeating what others have said.
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