That’s can be handled properly with only little problems, starting with the way how birds and pigs lost limbs between ABM and ABO. :)
The hardest part is answering the question “Why the Flock, after rescuing their island and “lived happily ever after” could transport on opponents territory.
From the movie we know that:
-All main birds (except Luca and Blues) know each other in some way,
-The prequel ends with Blues’ birth and all pigs are actually in the movie, so it can’t be much earlier some years before ABO (also shows that Bubbles is older than Blues)
Other flaws in Assembling the Flock:
-How Blues’ parents let them go so quickly (Red’s influence?)
-Hal and Bubbles, which should get to the island on their own (or at least Bubbles, because he didn’t see Blues before Halloween)
-Stella’s jumping between all the islands (Imo Bird Isl. > Golden Isl. > Piggy Isl.)
-Terence obviouly teleported himself to Bird Isl., but he needed to say Red that he’s his brother at some moment.
-Mighty Eagle? From hero to zero and again to hero?
Imo the original Flock, after events from ABM, came on the Piggy Island to be sure, that Pigs will never come back to Bird Island (by destroying their contraptions each time they’ll become too powerful). Then, Red found 3 eggs on the beach (because eggs just lie there, check Silver’s backstory). That’s good to start.
Or we should just assume that not everything can be matched in Angry Birds for now. ABM2 would say a bit more, but I’d love to see it as The End of the series, where birds from 3 islands finally defeats the Pigs for good.