Were the childhood scenes from IT Chapter Two filmed at the same time as the first film?

Question

There are a number of flashback scenes in IT Chapter Two (2019) which show additional details about the events 27 years ago. As I watched the film I noticed just how much the kids looked just like they did in the first film, which is no small task considering these are adolescents who would have been 12-13 three years ago and are now pushing 15-16.

So it makes me wonder if the creators simply had all their scenes for the second movie planned and written already and filmed them all at the same time as the first film. Has there been any indication or acknowledgement of this as being the case?

Answer 1

Wikipedia states:

Muschietti confirmed that two cut scenes from the first film will hopefully be included in the second, one of which being:

(Spoiler)

the fire at the Black Spot from the book.

IMDb states:

The young actors who were the Losers Club in chapter one grew tremendously in the 2 years following filming. They had to be digitally 'de-aged' in some scenes as they looked significantly older than before.

So the scenes including kids in Chapter Two were either unused scenes from the first chapter, or shot during the production of Chapter Two with some tricks used to hide the fact that the kids aged.