Keep the old URLs unless there is a strong reason to change them, because every URL change adds redirect risk and forces signals to resettle. The default in a migration is preservation, not improvement. A URL that already ranks carries accumulated signals, inbound links, and a stable place in Google’s index, and changing it means redirecting, hoping the redirect passes everything cleanly, and waiting for Google to reprocess and re-associate those signals with the new address. That process is rarely perfectly clean, so each change you make is a change you have to get right. Preserve first, and change only where the case is genuinely strong.
The reason to default this way is that every change adds risk for an uncertain reward. Redirects can break, target the wrong page, or chain in ways that bleed signal, and even a flawless redirect introduces a period where rankings wobble while Google catches up. A tidier URL structure feels like an improvement, but tidiness alone is not worth resetting a ranking page’s standing. The cost is real and immediate; the benefit of a cleaner slug is usually cosmetic. So the burden of proof sits on the change, not on keeping things as they are.
That said, keep is a default, not a ban. Change the structure when the new one delivers real, lasting value: a structure that genuinely fixes a broken hierarchy, supports a needed expansion, or resolves a structural problem that limits the site. When the gain is real and durable, make the change and then redirect meticulously, mapping every old URL to its true equivalent and verifying each one. The exception is earned by value, executed with care.
To decide, go URL by URL and ask whether changing it delivers real value or just looks neater. Keep every URL where the answer is “just neater,” and change only the ones where a concrete, lasting benefit justifies the redirect risk. Where you do change, build and test the redirect map exactly. Default to preserving URLs, and let real value, not tidiness, be the only thing that overrides that default.