You could see it as the whole international flight system being refactored, consumers will still use planes like before

We can pontificate on how to define the scope of a system here. I will only state that, from the perspective of a consumer, you could consider this a Service on which the interface of find flight, book flight, etc. would appear to be the same while the connections internal to each of the above modules would have to account for the change.

Functionally, I suppose it's the equivalent of upgrading an ID field that was originally declared as an unsigned 32 bit integer to a wider 64 bit representation. We may not be changing anything fundametal in the functionality, but every boundary interface, protocol, and storage mechanism must now suffer through a potentially painful modification.