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Joe I agree with Shane that it is very possible to keep rankings after changing domains, but that is if everything else stays the same. You are moving to a totally new platform so not much is going to stay the same. I would not expect rankings to stay the same after the switch. Still, if you must do it, make sure all of the 301 redirects are in place and working correctly. Do the best you can with what you have to work with. I'd advise against switching if most of their revenue comes from Google organic search. However, some retailers actually find Amazon to be their leading channel (over search, even) so it all depends on what their business goals are. Good luck!

Brilliant, this is exactly the answer I was looking for. Doesn't matter if it's only a small amount passed on, I think targeting this one over the other two will be far more beneficial.

Hi Joe, Welcome to the community. In my opinion, be very careful of using this plugin especially with Google going aggressive with their over optimization penalty. It does not look very aesthetically appealing to have visitors looking at a bunch of search terms at the end of your blog posts.