changes from when clause to if clause

Both are correct, but the meaning changes.

When I am ill, I go to the doctor.

This sentence implies a decided course of action, when you are sick.

If I am ill, I go to the doctor.

This sentence states that in the case of a decision you make in the case that you are sick.

If I am ill, I will go to the doctor

This is in a specific circumstance, not repeated. You would more than likely tell this to a concerned friend, if he/she sees that you are not feeling well.