Hi David,
This is really a Page Forms, not Cargo, question, but yes, you do need to modify both the template and form pages whenever you modify the data structure. I'm working on simplifying form pages, with the eventual goal of not requiring a form definition at all and just getting everything from the template definition, but that might be a ways off.
There's also another option, which is to use the Page Schemas class, which lets you define each "class" in just one place:
This might be overkill, though, depending on your needs.
-Yaron