Can't you just say the character has made a life change, or something? I know it's cheating and all, and fewer people will know who you are cosplaying immediately, but it shouldn't be that big a difference, considering such a short time frame to deal with.
And shouldn't this be in the cosplay forum? They would know better than I, a non-cosplayer.
Does renting wigs sound as gnarly to you as does me?