I think there's no harm in doing so.
Of course, only you know if you worked hard enough for an A* to begin with, so only you can decide how probable is an increase in your percentage. Plus you are only a mark away from an A*, so even if you get a decrease in your marks it won't affect your grade much.
I'm not too sharp on how Edexcel works though, so it'd be best if you got a second opinion.