Also not a network engineer. But I'd imagine first the Tier 1 ISPs (Level 3 etc) could enable jumbo frames and make sure they work smoothly amongst each other. After a few years of waiting, new consumer devices/OSs can start trying to send jumbo frames, while setting the Don't Fragment flag. If some router in the path doesn't support 9000, it will signal back ICMP Fragmentation Needed, and the device can transparently resend with 1500. Then add some local caching to avoid the roundtrip for subsequent connections to the same noncompliant network/endpoint.

And since it's a big change anyway, why stop at 9000? Why not 65000? Packets don't have to fill the entire MTU after all. If 65000 is supported, you can choose to send 1500, 9000, or 65000 based on your desired latency/throughput tradeoff.