Very easy to remove. All you need are arms that are a cross between Mr Tickle's and Inspector Gadget's, the hands of a child, the dexterity of a surgeon, the strength of the Incredible Hulk and the patience of a saint.
There fairly easy to remove, refitting just takes time and patience. Suppose the top seals have failed.. David gets them rebuilt using the later Ferrari double lip design seal which is a lot more durable.