Tockwith Training:
Seems a bit strange, but if that’s the rules then fair enough.
When I think about it, it’s no different than say a HGV class 3 driver who passed the test many years ago and has not driven one since the test - both have a licence and neither have passed the modern LGV test in it’s current format.
To my mind that makes both the C1 on grandfather rights and the HGV class 3 driver mentioned above at the same level in regard to the driver cpc.