'Growers' - young birds between about 12 and 18 weeks - do always go through a juvenile moult, shedding their chick feathers and growing stronger, warmer adult ones. I expect yours are still throwing a few old feathers, quite normal - and, because you have them at a time of year when the light is declining fast, they probably won't be in any hurry to come into lay and may even delay this until after the winter solstice, in the New Year. In Spring or Summer months, 19-20 week-old hybrids would be about to lay, or already laying, because the hormonal trigger of abundant light would have done the trick. This may disappoint you, waiting for eggs, - and they may yet make it into lay before the end of the month - but if not, they will grow on steadily and slowly and be good strong birds who will lay brilliantly once they get going in a few weeks' time.
At their age, I would just finish up whatever growers pellets you have, and then start them on layers pellets once it's gone. Maybe introduce the new food gradually by mixing it with the old for a week. No need to buy any more growers feed at this stage.