I don't see how installing a transfer switch -presumably for bringing in power from another source, such as a generator- would affect the downstream distribution arrangements.
Now, here's how I see the topic ... keep in mind that the NEC hasn't quite come around to this perspective :
The real question is: where does the 'service' end and the 'distribution' begin? When there's only the PoCo service, the answer is easy: at the first ("main" fuses or breaker) overcurrent device.
To me, thiis suggests that every power source needs to have some form of OCPD, and that your 'service point' would then technically move to where the different 'services' come together. That is, at the transfer switch.
(Keep in mind that I also advocate 'separately derived systems,' also known as where you also switch the neutral.)
Even if you are using some other form of generation - say, solar panels - where you actually want to be able to back-feed the PoCo, I really like the idea of a single means to disconnect the 'distribution' system from ALL power sources. In such an arrangement, I can see defining this disconnect as the 'service point.'
Farms are allowed a slight allowance by the NEC, where you can have a single disconnect serving all the buildings. Fari enough ... but, as I see it, the feeds to these other buildings is part of the distribution system, and an issue separate from the transfer switch issue.
Only recently has the NEC made plain that such feeds ought to be 4-wire (both neutral and ground). With an older arrangement, I do not see the installation of a transfer switch as requiring that the entire distribution network be brought up to date.