In short, you are not obliged to select KIPO as the IPEA even if you already designated KIPO as the ISA. ISA selection and IPEA selection are separate.
Currently there are 7 different IPEA's available to the US PCT applicants, that is, IP Australia, EPO, IPAU, JPO, KIPO, Rospatent, and USPTO.
But EPO is available as the IPEA only when the international search is or has been performed by the EPO, the Austrian Patent Office, the National Board of Patents and Registration of Finland, the Nordic Patent Institute, the Spanish Patent and Trademark Office or the Swedish Patent and Registration Office. IP Australia has a similar approach. Thus you have two choices. Either KIPO or USPTO as the IPEA if you used the ISA/KR.