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I can understand your situation Josh..!! But, as per my opinion, You should show the clear picture to your client. Be frank with your client. Tell him statistics that If we do this kind of things then it will be resulted in this. Make your client aware about how big changes you are going to do. OR You can give recent example of Moz.com. They have done something similar things. They have made 301 redirects for all seomoz.com URLs on Moz.com. You can see its current PR and DA and all kind of things. So, Collect such data and show your client the realistic view.