Not much changes.
For the first two days after cancellation, you will still have full premium benefits. Just in case the cancellation was a glitch in accounting.
After that, KeePassium will switch to the free version for casual users. (The app usage timer was disabled while you had premium subscription.)
Database Lock timeoutwas set to more than 1 hour, it will be reduced to this limit. So you might find yourself entering your master password more often.
Of course, you are always free to take your databases and switch to any other app.