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This is exactly right and is a great answer. Canonical tags stop content duplication from being a problem and can alleviate content duplication related devaluations (or in extreme cases, penalties) What canonical tags don't do anywhere near so well (if at all) is transfer SEO authority from one page to another. If OP did what they were suggesting, the risks would be (1) Google interprets the canonical tags wrong (2) Google starts ranking pages on the new site instead of the old pages, but (critically) without any appended backlink equity (3) all rankings are then lost on both sites I'd be extremely, extremely hesitant to deploy in the OPs specified manner and I think that Nigel is 110% correct here