If it’s a number issued by the International Maritime Organization for life, how would you do that? You can’t make up a number, and if you use a number used by another ship you’d have to make sure that your ship matches exactly the ship registered under that number so that you can fool anyone checking - and even then, that’d probably buy you a short amount of time but you’d still get caught.

How do you get a number assigned in the first place? I'd assume you'd have a shipwright just go through that process for a hypothetical ship they have "under construction" exactly matching your ship, then never build that ship, but instead assign its number to your ship.

IMO is assigned to the hull and can't be changed.

MMSI (a different identifier) can be changed when the vessel is sold or re-flagged