Bobajob
The letter should give you a date by which you need to start a claim for PIP, I'm afraid I can't comment on the CAB and what they might do but there have been significant cuts to CAB services so you may not be able to make use of their services.
The criteria for PIP is significantly different from that for DLA, so that probably makes the form more complicated.
Have a read of our PIP Claim guide for more details about the process and criteria you will need to meet.
www.benefitsandwork.co.uk/help-for-claimants/pip Gordon