Making Shuri BP takes no retconning at all. Merely requires her to grow into a role she should have been raised for to begin with. Regardless of whether or not T'Challa ever actually dies again, she should be prepared for this especially after Killmonger. This isn't a retcon so much as it's simply writing the royal family intelligently. T'challa has died twice ... they should be smart enough to have a viable line of succession through and beyond Shuri.
But again, if Coogler isn't done with T'challa he can recast. It's up to him.