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 :)