Honestly, would they even need to rebalance act1a at all? It'd make the act a bit easier sure, but this act is the introduction and therefore the difficulty increase would be more gradual I'd imagine. Act 2 and 3 would possibly need rebalancing but those acts would be expected to be harder so difficulty increases would work.
And like I imagine the game wouldn't be much harder with four party members instead of six. Since the difficulty range would likely be intended for 3 to 6 characters. Especially if they stay closer to base 5e in monster structure with their homebrewing in line with base 5e difficulty.
In few words: not really. But this is ANOTHER thing that was already discussed ad nauseam in this very thread (well, in several separate threads that were merged in this Frankenstein-one, at least).
For a start, ironically enough many of the fights feel FASTER and smoother played with more characters in your party. You have more roles at your disposal, you can afford to evaluate more tactical options (i.e. giving up on inflicting direct damage is more a viable strategy when you are not responsible for at least 25% of your party output), you get rid of enemies quicker and moving more "pawns" on the board has the side effect of making you spend less time just watching the AI mobs (that usually outnumber you considerably) doing their things.
On top of that, the mere goal to achieve some universally accepted "balance" in this type of game is utopic in itself. ANY degree of intimate knowledge of the system will always translate in a massive boost in efficiency. That's why some of us are just curb-stomping most of the encounters no matter which setup we are sporting, while there are people weeping that they need the "story mode" difficulty option because even the tutorial fights are wiping the floor with their asses.
As I already pointed all the Infinity Engine games were created with a party of six in mind and yet plenty of players played them with a smaller party that simply increased in level faster and few were even crazy enough to experiment their most overpowered builds in SOLO mode (at the maximum difficulty, too). Seems like everyone got what they wanted there. The beauty of implicit scalability. There will never be a "perfect balance" that will satisfy everyone. As long as they can test for the extremes (i.e. be sure that the hardest difficulty mode in the game is still reasonably doable) every other compromise below that is peachy and dandy.
I, for one, am not even particularly interested in the challenge just for the sake of it. I just want fun and engaging fights, but I have no intention to play at anything else than "core rules" mode, like D&D is supposed to be played. Any increase in difficulty that could come from "buffing damage and HP for the enemies" is garbage as far as I'm concerned and I would not play that. And the same goes (in the opposite direction) for some condescending "story mode". Even in Pathfinder (both of them) any other option than "core rules" doesn't even exist for me.
But even conceding that some work would need to be done to hit a sweeter spot, here's the main points: 1) none of the current encounters should be considered "FINAL" to any degree. Every single fight so far is a work in progress, so worrying about the potential need to tweak them is either disingenuous or silly. 2) D&D is a system that ALREADY includes suggestion on how to tweak/scale the CR of most encounters according to party size and level. 3) as consequence of the previous point, altering the difficulty of battles wouldn't really require the inane amount of work implied. In fact, given development tools mature enough and given the systemic nature of the AI in the game, it would be for the most part absolutely trivial. Literally a matter of dropping a couple more enemies (or raising in level/swapping some of the existing ones, or adding some extra environmental factor) with few clicks, without even any particular scripting involved. If anything it would be more work to TEST for them than to include these changes. But then again, if the additions/changes are gradual and reasonable extensive parallel testing would be superfluous too. It goes without saying that if a party of 4 can manage six brigands a party of 6 could manage to face eight. Or to have a stronger leader in charge of them. And it doesn't matter if the two variations wouldn't match each other 1:1 in terms of perfect fine tuning.