User blog comment:Shinhwalee/Major Event Preparation Guide for Admirals/@comment-26662684-20150811085831/@comment-26336020-20150821130522

If you haven't found out already, no you cannot change once you complete the map. However what some people do is farm E1 easy, if the boss hp gets to low, change to medium or hard and change back to easy to reset the hp. Then after you're satisfied you can change to hard and finish, which at that point it stays as hard till the event expires.

Of course the downside to this is that you have to sit on a lower map for a longer period of time potentially not finishing the event if that was your goal. Also Ooyodo can be farmed on 1-6 later, so please take that into consideration.