James
Your ESA(CB) claim should be closed by the DWP automatically although they do not do it immediately that ESA(CB) payments cease in case the claimant makes a claim for ESA(IR).
I would write to the DWP office that has dealt with your claim saying that you wish to close your ESA claim and any ongoing NI Credits only claim.
There will be no requirement for you to attend an closing interview or medical.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems