Vrillon
Given the timescale I would tell them at the assessment, write a short letter explaining the changes and in particular any changes to your abilities to complete the ESA activities. Give the letter to the assessor at the start of the interview and ask them to read it before they start and to include it in your file.
I would also send a copy to the DWP office dealing with your claim, the address will be on any letter that you have had about your ESA.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
