Y'know, people keep saying this.... but it sure isn't written anywhere. The AL website has a great article by Art S which covers a lot of useful ground, and doesn't agree with the above assertions. I realize there is a penchant to try and have a centralized control of the game, because sometimes people are afraid that things will go out of control otherwise. But that does not seem to match the actual AL approach as indicated in the ALPG, the AL website, and posts by the Admins.
In general terms, the mentioned article takes the same approach as the ALPG....to try and stick with the written adventure, but to make changes as needed to make sure it is fun for the players. There are actually very few things that must stay exactly the same.
In more specific terms, the article says you *are* allowed to add encounters, but to be "very careful" and "mindful of the time". Similarly, you are not limited to just 'more of the same monster' as keeps being asserted. It refers (in various places) about adding creatures of the same "type". That is *not* the same as the 'same creature'. 'Type' has a very specific definition in DnD, particularly when referring to monsters/creatures. They don't want you swapping out some fiends for undead, or dragons. But if there are basic skeletons, you are not limited to only more basic skeletons....
The AL clearly trusts its DMs to make the game fun for their players....the admins seem very consistent in this.