Only one Zoo Club subscription can be active at any given time. If you buy a different kind of ZOO Club subscription while you already have one active, one of two things will happen:
If you buy a lower ZOO Club subscription than the one you have, the new one will be activated after the current one expires.
If you buy a higher ZOO Club subscription, then you get a bonus - the new one is activated and the time remaining in your lower subscription time will be added to the higher one.
If you buy a lower ZOO Club subscription than the one you have, the new one will be activated after the current one expires.
If you buy a higher ZOO Club subscription, then you get a bonus - the new one is activated and the time remaining in your lower subscription time will be added to the higher one.