2 notes filed under migration, newest first.
Move it piece by piece, unless the whole product is small enough to rebuild in one short project while feature requests wait. A rewrite stops your releases until it is finished. A piece-by-piece move keeps them going, and if you stop halfway you still have something that works.
List every old address before anything is designed, give each one a permanent redirect to its closest new page, change one thing at a time, and watch the numbers for ninety days. Nobody can promise rankings. Anyone can account for every URL.
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Free tools