When you install an Admin System certificate on any ISE node, the application services will restart and that has an impact if that node is the Admin GUI (PAN ... loss of admin UI) or if the node processes RADIUS/TACACS (PSN services). However, if the WLC has more than one Server configured, then the loss of one of these servers will just cause a slight delay until the next Server is selected and then used to process RADIUS/TACACS.
If you are replacing an EAP System certificate, then you have no impact.
