Hello,
Yes you do have to remove the filler as it goes into the back of the toilet.
The toilet is secured by 4 or 6 screws under the cassette.
So, remove the filler , undo the screws, inside lift the toilet a little and pull forward.
Oh ! nearly forgot, check to see if the toilet has been siliconed along the edge of the cassette door.
If so, you will have to cut that then remove it all and renew on refitting.