Why would you have to retopologize it?  The problem is with the UVmap, that it is not "flat".  You'd have to redo the UVmap, which makes any existing textures invalid (they won't line up with the new UVmap) or you can get into texturing with some program that does projecting painting - however for any model of a curved surface, some amount of distortion is inevitable unless you map it with a very large amount of seams.

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