Gosh, this is complicated coleoptera.
So... when you fill in the bit for DTs , it asks for household income. If that is over about 65k nothing happens. If under, DTs are asked for names of people who need to provide further financial info.
You then get an email. You have to provide details of income for tax year 2017/18.
If it is projected to go down by more than 15% then you have to provide income for 2017/18 still but then fill in a form (after googling for three days you will find the right one) detailing your projections for 2019/20(with your crystal ball in hand!)
I don't think there is any form you will fill in if you think it will go up, but am guessing if that happens when you provide details for the next year (ie second year) they claw back overpayment (from your DTs) at that point. Hence my jitters.