Hello 계시다 is the honorific form of 있다 In this case 이다 cannot replace 꼐시다 If you want use 이다 in a honorific way, you can use the honorific maker 시 ~이십니다 (Very formal) Or a slight less formal way ~입니다 (Formal) Another verb example is 먹다 = to eat. The honorific form is 잡수시다 Thank you :)