by SteveK2
Technically, you remove all old tiles, then place new tiles. The rules mention that some tiles (Ship, Mine, Castle) are replaced by identical tiles, so you're doing unnecessary work by replacing them.However ... this "don't bother" comment isn't always correct - in a 3 player game one space alternates between Castle & Mine in different turns, so here you would have to remove the old tile & get a suitable new tile.