You have to just run youtube-unsubscribe-all-script.js script. After removing all subscriptions it prints them in the table, so you can easily go to these subscriptions that you want to subscribe again.
In order to use the script, you have to go to youtube subscribed channels, open developer console (f12 > console tab), print script and press enter. It will remove first 100 subscriptions and print result. After that you have to run this script again, and so on if you want to remove all subscriptions.