You didn’t actually lose the extensions. Unless you deleted the folder they are in for the older version of SketchUp, they are still there. Each version of SketchUp installs as a separate program. It’s not like some other software that overwrites the old version with the new.
If you copy your extensions from the older version to the new one be prepared to chase load errors. Some extensions may need to be updated to the latest versions and some will require that they be installed correctly, not just copied from one folder to another.
I expect that feature would be added if they could somehow force users to keep their extensions up to date.
If you install the latest version of the Sketchucation ExtensionStore tool first you can use it to install your bundle of extensions from Sketchucation. You can go to My Downloads in the Extension Warehouse window and install the extensions you got from there. Doing that ensures you have the latest versions of them. It also gives you a chance to do some housecleaning and get rid of extensions you don’t use.
That’s not a new thing. Go into View>Toolbars and turn off Large Icons.