If you have to transfer from C to N, or Vice versa, you’ll have to take the train, but it’s not that big of deal. The airport is designed so that you can arrive at any set of gates and depart on the other end of the airport with relative ease. The only semi challenging connection is over to the international gates (S gates). Everything else just requires one train, which is downstairs. You’ll likely be assigned C, N, or D gates, and any combination of these connections is very easy. As an aside, the N gates used to feel like purgatory, but now are very nice!