Polymer v2.4 is out and it has some changes to prep you for what is coming in v3 (for example, template changes for components). However, if we assume that the v3 “Modulizer” does its job, you shouldn’t have to upgrade because moving your templates into the classes should be handled automatically later on.
The blog entry also covers some of the progress toward v3 and also mentions that there’s a new 3.0-pre.6 available now.
If you’re curious about their progress toward the next version, you can tech out the website where they track progress on converting their own suite of Polymer v2 components to v3 automatically with their conversion app I mentioned earlier. They seem to have gotten over the hurdle of being able to convert every single one, however they don’t all yet pass testing.