What they will be able to do once the paperwork is filed, is to ask permission to appeal the making of an adoption order. They can't automatically appeal the making of the order, they have to go through an extra bit first. And indeed, the vast vast majority of appeals fall down at this stage - the Judge does not grant permission for the birth family to appeal the making of the order. The remainder fall down at the next stage, however many court dates there are.
The law does theoretically allow for a child to be returned to their birth parents at this point (by succeeding in getting permission to appeal, then successfully appealing, appealing the placement order, then the care orders) because it has to give everyone the right to appeal. However just because the possibility exists, doesn't mean it ever happens. To the best of my knowledge, no child (who wasn't relinquished) has been returned to their birth parents at this point (by the birth parents starting at the point of asking permission to appeal the making of an adoption order). I can think of a case of a (removed) child being returned after being placed for adoption, but the legal situation was very different.
The judge decides what significant change looks like, after hearing the birth families evidence, the judge decides whether or not it's good enough.
I do understand that it's really worrying, and so many people worry about it. But truly, it's not something to worry about xx Have your childs birth parents made major life changes? It's very uncommon. And even if they have, the court has to take into account the childs best interests - they have to weigh up the damage they could cause by removing a child from their adoptive family, and a court will be very reluctant to do that to a child at this stage, unless in exceptional circumstances.
Hope that is helpful, please ask if I'm not making sense. I know 'not worrying' is much easier said than done, but the majority of birth parents ask permission to appeal at this stage and none have succeeded to the best of my knowledge, and that speaks for itself.