> has to have their networks reconfigured.

I don't see why every user couldn't be auto-created a virtual network with the same 10.x.x.x IP addresses as their original machine had - and therefore there is no need to do any reconfiguration on the users side.

Because there's more than just the local IP address to worry about.

Remember, all of EC2 Classic was in a single /8 of private IPs. You could communicate with EC2 instances in another account via their private IP address.

If you have two instances in different accounts that need to communicate, upgrading from EC2 Classic to VPC couldn't be done automatically.

Because people could have that same network on-promises on the other side of the VPN (I have).

But that isn't a new problem if the same user already uses that address - you're just leaving them with the same issue they already had.