Also, will I need to rebuild shaders for assets that I've already created or is it a seamless transition?

They could have made it clear in the marketing materials, but the OpenPBR shader is built for MaterialX 1.39.0, which is in-progress and unreleased:

https://github.com/AcademySoftwareFoundation/OpenPBR/blob/main/reference/open_pbr_surface.mtlx#L2
[github.com]

I can't say when it would become the default, but I think it's safe to assume that won't happen until MaterialX 1.39 is finished, and Houdini adopts that in a future release.

As it's a new shader, so you'd have to update your materials to use that shader; it's not a behind-the-scenes change.