As there is no LEDE 17 or OpenWrt 18 stable build for C2 v1, I think you must have previously installed a 'custom built' LEDE 17 snapshot build. This may be why sysupgrade is not happy to upgrade to latest development snapshhot. Someone will correct me if I'm wrong.
You could try forcing the sysupgrade with -n -F flags, BUT there is also a high risk of bricking the router! You may then require a serial connection to help recover/unbrick the router in worst case scenario.
I would suggest waiting for another member who may have more knowledge of C2 v1 to offer more advice before considering a risky forced sysupgrade option.