Transfer from DLA to PIP.

I received a letter last week asking me to apply to transfer to PIP by 18th July. I have Higher rate for care and mobility indefinitely for 8 years. Today I had a text reminding me to call. (I didn't know they had my mobile number) I need to have a number of things and information available when I ring.
I have been dreading this. Is the call just to request a form to complete or will they ask me details about my condition? I would prefer to put things on paper, when I have a chance to think rather than answer questions over the phone. Also I moved house 2 yrs ago, and have a new GP surgery. My old GP retired anyway, but he knew me and my health problems. I have not seen the same GP in this new practice, and not for the issues I am given DLA for (apart from medication review) , as I see the Pain Clinic consultant. I will obviously put his name down, but would appreciate some advice before I start the dreaded process.