Like Punx lots of people on the SN boards have said DLA to PIP as appointee was a fairly seamless exercise and others like me have had a terrible time and have had to go all the way to Tribunal.
The CAB online guide is free, the Benefits and Work guides are downloaded but you need to join them for a year approx £20.
Surely it's a lot easier to go from DLA to PIP even though the criteria are different, to applying for PIP for the first time
It wasn't in our case, DD had indefinite DLA then after the F2F was awarded lower Daily Living but zero mobility PIP.
I'd strongly advise you to use the guides to explain about the tasks they mention and stress when your DS cannot do those things reliably, repeatedly, safely and in a timely manner those words are key.
In the F2F or whatever's in its place now, answer questions 'No, but'
Never say 'Yes, but' as they'll only hear the 'Yes'.
Get as much supporting evidence as you can.
Be prepared and expect the worst from DWP, then you'll have a better mindset to deal with it if it happens.
I was sent a refusal letter and it states you have to appeal "within a month of the date of this letter" only it didn't arrive until 2 weeks after that date plus the remainder of time included a Bank Holiday, so the fictitious 'month' was only around a week in real time.
Send all correspondence Post Office Next Day Tracked, it's about £8 per time, but if you, like I did, receive a letter saying you'd not submitted something within the "month" therefore they were going to declare your claim out of time, you log onto the Post Office website and screenshot the date and time and signature of the person at DWP who received it and reply with that evidence. DWP never apologise.
Submitting a diary of the challenges your DS faces is a very good thing to do, it demonstrates the differences between him and his age peers.
Lots of people online will tell you to document each day as though it's the worst day, please don't, just tell the truth.
Each knockback from DWP comes with a warning that if they have to reconsider, you may be scored even less points than your original award. It feels very intimidating, so many people give up at that point and settle for a lesser amount out of fear they may lose even that. Grit your teeth and put your Big Girl Pants on, make sure you refute their dismissals and counter them.
Our Tribunal Judge said our case was extraordinary, inasmuch as he, the medical expert and the disability expert had all independently read all of our evidence and all independently reached the conclusion that DD should have been awarded enhanced rate of both PIP components on my original submissions alone. He said if DWP had bothered to send a rep he would have made them apologise to us and he awarded PIP without a time limit using the phrase 'It is inappropriate to set a term' which means she will only get a very light review once every 10 years.
It can be a very unpleasant experience to go through, but as long as you understand what information they need and how to present that information so DWP cannot dismiss it, you'll succeed.
I'll keep my fingers crossed that you have a really smooth transition, but in case you don't, I hope my experience is helpful for you.