You can basically switch for "free" when you get your third civic point, and in the early game most planets aren't even developed enough to build a single branch office building on them. If you want to form a trade league you can do it with the merchant guild civic too.
In the early game, it can better to
start as a Megacorp and reform
out of it when you have your third civic. This is because most normal empire civic bonuses only really start rolling in the later-early game, while Mega-Corp has some substantial bonuses for the immediate early game that can provide substantial assets in that period for you to get the tech and reach that third civic stage in year 40-50 or so.
If you aren't mil-rushing as a genocidal, one of the strongest civics in the early colonization phase is the Private Prospector civic, which lets you buy colony ships for 500 energy instead of the 200 alloys/food/consumer good. This is a
substantial savings in all the resources, not only saving you in the resources themselves but
also the pops needed to
produce those excesses. Instead of employing another Industrial District or 2 of workers producing alloys and CGs for nothing but colonization ships at a reasonable rate, you can use those workers for your science program, while stockpiling the not-used alloys for your fleets (2 corvettes or 1 star base each colony ship early game), or not producing excess food you don't need so you can employ another farmer instead of miner.
This is
far, far more than the CG/Alloy/Science benefit you might get from, say, Meritocracy as a normal empire. Meritocracy's 10% specialist output is considered great, but adding just one more scientist to your empire outweighs Meritocracy's entire science benefit until the point of 10 scientists, which you can't afford in the early game. For Meritocracy to match 600 alloys from just 3 colony ships, at roughly .3 additional alloys per 3-alloy-per-job workers, you'd need 2000 job-months, which would take 10 alloys workers nearly 17 years to meet at a point where you probably aren't able to afford 10 alloy workers. And of course the food savings is just straight-up a worker pop that could be doing something else rather than building a stockpile. Like, say, being a scientist for another greater-than-10% scientist increase.
This is a very potent economic, military self-defense buff at a point in the game where most conventional empire expansions entail accepting risk that you're not going to have hostile neighbors, and trying to balance feeding their expansion for long-term gain and feeding their science economy. Whereas prospectors can just... employ the scientists on the CG they don't have to keep stockpiling, are able get more colonies established sooner without having to wait for the stockpiles to build (and thus get use of their colonies to start building), and get the tech to reform out of mega-corp before 'better' civics come into play.
Add to that the Ruler Pop increased energy income in the early game (great for buffing your early mineral economy with monthly purchases, also freeing up pops), 30 admin cap (a significant buffer delaying the need for more admin guys to avoid net science loss until later, although you do face steeper penalties after the point), and a few other civics for strong early-economy (+4 starbases can be paid for by the colony ship savings, and have hydroponics bays that remove the need for
any farmer jobs until you have 80+ pops, letting your guaranteed worlds be purely energy/mineral centric; alternatively catalytic processing can make use of the significant spared food while alloying your mineral economy is able to afford more scientists sooner),
and the ability to form the best Federation early in the game
without having to spend a civic slot on the option...
Mega-Corp
starts have very strong potential, and their biggest weakness is unity production. Which- at this time- is one of the least significant resources, and can be mitigated by their own early-game economy civics.
