For anyone else who comes along:
The new migrator copies the extensions from the previous SketchUp installation. If you haven't stayed on top of keeping extensions updated or if extensions required updates for the new SketchUp version you will still need to go and track those down. And for extensions that have their own installers, most likely you can expect those extensions to fail to load and you'll need to install them correctly.
Even if you use the migrator, the very next step regarding extensions from Sketchucation should be to open the Sketchucation Extension Store tool and install the bundle to make sure you have the latest versions of extensions from here. That will also allow the tool to alert you when there are extensions that need to be updated and it will create a new bundle for later use on other machines or for the next major release.
And don't get lazy about keeping extensions up to date. The authors do their best to keep them updated. The least you can do is take advantage of their updates.