Other than moving to npm and es6 modules Polymer 3 is exactly like Polymer 2 and feels like home. The API is the same and does not require any change and again if I recall correctly the Polymer team said they are even building a tool for migrating Polymer 2 to 3.

Polymer 3 has a lot of potential to be the perfect library, it just needs the right tooling to answer the community needs and of course the most important thing to keep everything simple as it always have been with Polymer.

I would like to thank the Polymer team for their great effort making this amazing library and the tight communication with the developers.