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you should normaly delete and then reupload the photos that you allready uploaded with gd1 if you change it now and the pics where uploaded gd1 will not affect the pics!so you must reupload them!
more exactly upload an new pic that is not in your hosting and see the result with gd2,if you like it delete the pics with gd1 and batch upload them again with gd2!
logicaly the new ones will be improved!np