My son is currently going through the process of Child to adult passport (which is what we are waiting for). His passport expired this month (May 2022) and when I was filling in the documentation online for this I was told that this application was classed as “Adult Renewal”. HMPO requested his child passport for his “documents”. I submitted the application on 25 May, documents were marked as received on 28 May and today I received notification that his application had been approved and the next step is printing.
From what I have been able to find out in the meantime is that if you have lost your passport/it expired a while ago then you “may” have to go through the interview process for a new adult passport. This is not always the case.