DHCP server subnet input validation needs to check config.xml, not ifconfig
The DHCP server subnet input validation in 2.0 checks what IP is configured on the interface, so it's impossible to change the subnet and DHCP at the same time. You have to change the subnet, apply changes, manually IP your client system, connect up to the new IP, then change DHCP. In all previous versions you could change the DHCP server subnet after changing the LAN IP but before applying changes. Then when changes are applied you just renew your DHCP lease and you're back in on the new IP.
To fix, just need to change the DHCP server input validation to use what's configured in config.xml, not what's actually on the interface.