Thank you so much for your quick reply.
Basically, we have a clinet that already has a number of APs used as Instant. The client want to install a controller.
I would like to propose tot them two options. Option 1 will be the standard configruation by removing Instant and using the controller. Option 2 will be keeping the APs as Instant but anchor back to the controller for Guest access configuration. So, if the controller fails, then only guest access will be unavailable. Hopefully that makes it clearer??