I think it's a little more involved than that. Each port has dates when new boats become available. If you put into port after a good patrol, and it's right when a new boat becomes available, you'll be offered the chance to upgrade. It's a blind chance; you have no idea what type of boat you're being offered, nor will they tell you how much renown it'll cost to accept. But if you accept, you get a new boat.
Word of advice: if you're based in Pearl Harbor, do not accept an upgrade between Jan 1942 and the end of May 1942. There's supposed to be a conflict in the files where you can get offered a Gato there as early as April, but the game thinks the Gato isn't available until June, so when you return to port you get retired "along with the class".
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We, the unwilling, led by the unknowing, are doing the impossible for the ungrateful. We have done so much, for so long, with so little, that we are now qualified to do anything with nothing.
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