by timeodanaos
Neither the English nor the French rules appear to answer this question. If it might make a difference strategically, I suggest that a simultaneous decision would be the most appropriate (e.g. by revealing a die or token at the same time). Any other method of assigning priority (Tokyo vs Tokyo Bay, turn order) is arbitrary in the absence of a rule.To my mind, the rules seem to specifically prohibit 'slideover'. First, "Monsters in Tokyo City or in Tokyo Bay are considered 'in Tokyo'" and "Tokyo Bay acts like Tokyo City". "If a Monster is eliminated and that brings the number of Monsters below 5, Tokyo Bay can't be occupied anymore. The Monster in it must leave Tokyo." The monster can no longer be in Tokyo, i.e. Tokyo City or Tokyo Bay. This presumably includes the case in which Tokyo City is empty. In other words, if you are forced to leave Tokyo Bay, you must leave Tokyo (City/Bay) entirely rather than 'move up' to Tokyo City. From this we may conclude that once you're in, you stay where you are, and if you're in Tokyo Bay if/when it disappears then you're out.
I assume it's been noted before, but the 5-player example in the English rules has an error in it (the sentence in which the error occurs is not found in the French rules);
"If Mekadragon had abandoned Tokyo City Mekadragon Gigazaur would have taken that control instead and left Tokyo Bay unclaimed."