The plugins will be a lot more similar than the themes. The MyBB team has added a lot of new features that will be useful for plugin development in 1.8, but they haven't really done much to break compatibility with existing plugins. Some of the template modifications needed for plugins may need to be updated for 1.8, but besides that, it's only really making use of new features for 1.8 and changing the compatibility marker in the plugin.
I had attempted to import a couple of themes back with the beta 2 release. They import, but MyBB has changed so much since the 1.6 release that for advanced themes, it's probably best just to redo them. The MyBB community is essentially starting fresh as far as themes are concerned, but the new default theme gives a much better base to start from. That's for sure.
Anyway, as far as your custom theme, you're probably best off just doing a for-scratch port of it. As for plugins, it looks like popular plugins are going to either be ported, or have replacements coded for them pretty quickly.