It's the project file format that's changing from xml (capx) to json (c3p). The plugins aren't affected by that. Any changes needed to port a plugin over are likely just busywork.
Since c3 is primarily a editor rewrite, the runtime portion of plugins and c3's runtime such would be likely the same. Although any changes to c3's runtime would probably be a good thing.
The edittime portion of the plugins is likely the part that would need to be re-worked, and that's actually the simpler part of the plugin. Probably most of that portion of the plugin could be converted over automatically, and only edge cases would need to be tweaked. That said people like rex probably wouldn't have any issue converting his plugins over quickly.
At any rate, just let the beta come out and it can be figured out quickly. Scirra ported all the official plugins so I don't see an issue.