With respect to Calibre plugins, you might want
DeDRM - especially if you want to download the Kindle books that you've already bought from Amazon and either hold usable copies on your hard drive or sideload them onto your new Kobo. Note that if you want to do this, your Kindle must still be registered to your Amazon account at the time you download a book from Amazon to Calibre.
If you do want to block OTA firmware upgrades, there are one or two patches that you might like to apply. But, as @Sirtel says, you can always go backwards if you want.