Currently, the advanced network options are only available for new orders from Generation 12 onwards. They will gradually be extended to other plans in the future.
No, this change does not affect servers that have already been set up.
No, if you select "Cloud VLAN only", you will not be able to add public IP addresses at a later date. The server will then be accessible only via the private cloud VLAN. Order a new plan with the appropriate configuration in this case.
Once set up, it is not possible to change the selected network configuration for server products. However, with the "IPv4 + IPv6 Connectivity" option, you can add IPv4 addresses or /64 IPv6 subnets after setup for an extra charge. With the "IPv6 Connectivity" option, you can add additional IPv6 subnets.