[FullyFunctional]

There are two issues in migration: updating software and updating configuration. The former can be completely trivial if you are just pulling down updates from someone else. The latter requires effort on your part. If the IPv6 address space had been an extension, then you wouldn't have to do any configuration to support IPv6 clients while you maintain your existing 32-bit address, valid for both stacks.

At least that's my understanding of it.

[keeperofdakeys]

For a new transport layer protocol like QUIC, this can work. However when you start talking about IP, you need to update/replace basically every device on the internet. If a device in the path doesn't know about the extended address space, that packet probably can't reach it's destination.

[FullyFunctional]

? You have to upgrade the software on all devices to move to a larger address space. That's understood. What we are discussing here is the added complexity of the configuration. Updating the software is easy by comparison.