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Hey Danny, sounds like you have a big migration project ahead of you! As others have said, you're going to have to do mass 301 redirects to the site you want everything to live on moving forward. You should also engage with an international SEO consultant to help you out with this, because you're likely going to need country-specific subfolders and use HREFLANG to send the search crawlers for the right languages to the right pages and rank them accordingly. Happy to recommend some if you'd like.

Hi Danny! How'd that crawl end up? If Andy answered your question, mind marking his response as a "Good Answer?" It'll get him some bonus MozPoints, and it helps us keep track of things in the forum.