Choose by size and risk: a clean all-at-once cutover suits smaller, simpler sites, while phasing suits large or complex ones. The pivot is your site’s size and complexity weighed against how much risk you can contain. All-at-once limits the period of mixed signals by moving everything in a single, well-prepared switch. Phasing trades a longer volatility window and more complex redirect handling for the ability to contain risk and learn as you go. Neither is a default; the right call follows from how big and intricate the migration is.
All-at-once works best when the site is small enough to test thoroughly and redirect cleanly in one pass. The advantage is a short, defined transition: Google sees one change, reprocesses it, and settles, rather than living through a drawn-out mix of old and new. The cost is that everything rides on one launch, so it demands a complete, verified redirect map and solid pre-launch testing, because there is no second wave to catch what the first missed. For a tight site with a manageable URL count, that concentrated risk is usually the better trade.
Phasing works best when the site is large or complex enough that a single cutover would be too big to test reliably or too damaging if it failed. Moving in sections lets you migrate a slice, watch how rankings and crawling respond, fix what you learn, and apply it to the next slice, which contains the blast radius of any mistake. The price is a longer stretch of volatility, since the site sits in a mixed state for longer, and redirects get more complicated because old and new sections must coexist and link correctly throughout. For big or risky migrations, that controlled, learn-as-you-go path is often worth the extended window.
Size up your migration honestly. If the site is small and you can test and redirect it fully in one go, do a clean all-at-once cutover. If it is large or complex enough that one switch is too risky to verify, phase it to contain the risk. Either way, plan the redirect strategy to match the sequence you choose before you move anything.