Hi Clive
The entitlement letter is basically the letter that the DWP send you every April stating what the PIP/DLA rates are for the next year so your next letter will be for the entitlement for April 2023 / 2024 and your current one should be from April 2022 to April 2023. The letter will be headed something like “Personal Independence Payment Yearly Update” but in your case will say Disability Living Allowance.
As far DLA to PIP goes I thought everyone had been put through that mangle by now and if your still getting DLA it must be because of your age as there was a date where you kept to the old DLA if you were born before a particular date but I can’t remember what that date was now.