The legacy module isn't written to be a drop-in replacement for 2.x code - it uses the new 3.0 packages (com.sencha instead of com.extjs), and extends or makes use of a few of the classes in the main gxt.jar file.
The legacy jar instead provides access to some of the functionality from 2.x not otherwise available in 3.0, such as the ModelData psuedo-reflection interface and base classes, the Bindings/FieldBinding model to bind that style of data to fields, and the old MVC framework present in GXT 2. These classes are provided for teams that want to update their projects while changing as little as possible - not taking advantage of new developments like the GWT editor framework, or RequestFactory, or using regular POJO beans instead of extending specific GXT classes. The legacy jar is really only intended for architectural pieces that GXT 3 isn't supporting any longer, but may still be useful for some teams who haven't migrated to any of the newer ways to solve these problems more effectively.
Certain things aren't present at all, such as non-existent layouts (FitLayout, not needed since all containers that use one child always already size like this), or BeanReader (GXT 3 can already natively work with any class that has getters and setters exposed, see the PropertyAccess interface).
And it is possible to use both GXT 2 and GXT 3 in your project at the same time, and is even encouraged when migrating. As they have different packages, they won't collide, and most of the 3.0 features work with 2.x components. See the document and sample project at http://www.sencha.com/learn/running-...and-3-together for more information.
Thanks for the explanation, Colin. And that link is just was I was looking for but couldn't find. I'm finding that GXT related resources aren't very google-able because sometimes it's referred to as GXT (which I prefer) and other times it's Ext GWT.
I'm also looking for any sort of migration guide, which I cannot find. The package hierarchy is totally different, and I have to figure out (the hard way) what things moved where, and what to use when things no longer exist in 3.0. The only thing I've found is a slideshow from November, which caused my confusion on the legacy module.