If your company is that big and unwieldy that a tiny chance of needing to do a migration in 12 months is unacceptable, you can stick to the key services which offer 36 months of notice. For all I know at that point you can negotiate your own guarantee if you're so big.

If you work at a company that can't do a cloud infra migration in 36 months, I don't have any good advice for you, but I doubt most commenters on here fall into that bucket. Frankly, I would expect companies that large to be in all the clouds so that you could negotiate them against each other, but I doubt that impacts many people here, and the chance of any of these worst case scenarios happening is vanishingly small.