Does my departure tax valuation get locked in permanently, or can CRA challenge it later?

No, a departure-tax valuation isn't locked in permanently just because you reported it on your T1243 and moved on. Like any other figure on a tax return, CRA can review and potentially reassess the fair market values you used for the deemed disposition, generally within the normal reassessment period that applies to your return for that year, and in cases involving misrepresentation, potentially beyond it.

Because departure tax often involves valuing assets without a clear market price, private company shares being the clearest example, these are exactly the kinds of figures CRA is more likely to scrutinize closely if your return is ever selected for review, compared to a straightforward publicly traded security with an obvious quoted price. A challenge doesn't mean your original number was necessarily wrong, but it does mean you need to be able to support it with real evidence, not just an internal estimate made at the time.

This is why a proper, professionally prepared valuation done around your actual departure date, with documentation you can produce later if asked, is worth the cost compared to a rough number that might look reasonable at the time but can't withstand a later challenge. Keeping that documentation for as long as CRA could reasonably reassess the year is a sensible practice.