First, you do NOT need to turn off FileVault to upgrade.
The act of changing the FileVault setting takes under a minute.
Then, over the next afternoon to possibly the next day or longer, whenever your Mac is awake, it will slowly read every used block on your drive, and re-write it, using on-the-fly encryption/decryption as appropriate. You can use your Mac for 'ordinary' things while this transition is in progress, but you may not run software update.
Many things that take up space, such a Time Machine snapshots, are listed in some displays, but considered "purgeable", so they are not listed in other displays. If you have not recently connected your backup drive and allowed Time Machine to run, it would be prudent to do so now.
any confusion about how much space remains on your drive could also be an indication that your drive directory is damaged. It would be prudent to run disk Utility and repair. If you cannot repair the running system, restarting in Safe Mode can attempt to make the repair as it starts up.