The return disappears; the obligation does not. If you are mandated into MTD for Income Tax, you send quarterly updates and then a final declaration instead of a Self Assessment return — but the final declaration does the same job, falls on the same 31 January date, and carries the same legal weight. Your payment dates are unchanged, including payments on account. If you are not mandated, nothing changes at all and you carry on filing a normal return. It is also perfectly possible to be in both worlds at once for a while: mandated on your self-employment while still declaring dividends and savings, all of which land in the final declaration. So the mechanism changes and the shape of the year changes, but the destination is the same.