You need to contact your server administrator, if you want to verify that the change was legitimate.
Please see:
Warning – Potential security breach!

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Do you get the same prompt on new connection?

I am getting the warning "The server's host key does not match the one WinSCP has in cache...." that occurred while I had an open session. The indication is that the host changed the SHA-256 key while my session was open. I am concerned that this is a MitM attempt, but I am not sure. I don't want to update my key unless it is legitimately a host key change.