Ah, it was an ex-military vehicle. For those cars there isn't an alternative other than getting an EA plate, right?
IMO (silly as it might be), I think the registration is part of vehicle itself. Few decades down the line it would be great to see a car that you used to own running around - for most part, its likely to be recognized by the registration (unless it was a rare vehicle in SL). Changing the plates from say 300- to EA simply isn't something that I would go for if I was offered...
By the way, what's the Merc? A W140?