Many people stop being classed as self-employed and yet still receive a class 2 national insurance contributions demand letter quarterly.
Even if you are still trading but have changed your mode of operating from sole trader to a limited company you have technically stopped being self-employed from HRMC’s perspective.
So long as your accountant is aware of the change they should inform HMRC for you. Alternatively you can do this yourself quite easily either by calling HMRC on 0300 200 3505 or by filling out an online form.
Once done their systems should be updated and your demand, up to the point where your self-employed ended, should be scrapped.