Very unlikely to happen because of fees, but plausible if the customer is high-profile enough that news about the move damage the reputation of the service provider.
Usually the new provider will advertise the win in such a way it highlights their competitive advantages. This can be extremely painful for the company that lost the very high profile brand.
I'd put Netflix up there as a first candidate...
Are they still a pure AWS house? I'd expect that, at their scale, they'd have boxes and VMs everywhere the round-trip time justifies.
They use AWS for everything except content delivery which is handled by their Open Connect content delivery system that they built and operate. https://openconnect.netflix.com/en/
Interesting. For a moment I thought they would aggressively optimize encoding for frame cost but then I realized that's probably a rounding error compared to their costs in storage and co-located infrastructure.
They have boxes at every major ISP borders, but they're solely caching infrastructure.
They publicly advertise this under the "Netflix Open Connect" initiative:
Can any single customer smaller than the US government get a policy change at Amazon by threatening to walk away?