For those switching with very severely disabled children don't worry too much. My son (severely autistic, severely learning disabled, epilepsy, non-verbal) switched without problem. It's actually easier for people like him to claim enhanced rate mobility than it was claiming HRM under DLA. Although I have just discovered he no longer has an automatic entitlement to a blue badge
Unfortunately not for my son with autism/learning disabilities. He used to get high rates of DLA, but now does not qualify for PIP. I also, of course, cannot get carer's allowance either. His needs are the same as before, in fact as an adult now, in many ways more complex. Whatever anyone says he cannot work and he stlll needs care, just for no money now.
I do believe there is a proportion of professionals who do believe any ASD magically disappear on your 18th birthday. In our area of the country there is no doctor or clinic for adults with ASD.