I can only tell what I have learned so far.. A couple of months ago I had to phone the DWP. To inform them I had been in hospital,, that was ok you have to be in for more than four weeks..
I asked while speaking to them what happens to my claim wich end May 13 they told me to still send my renewal pack in as before,,,they are starting pip in April as you say but I was informed that it to start with NEW claimants only at first,,and only in, I believe the midlands
before it is rolled out across the country, I said as I'm medically unfit for work and legally retired, why would I need to be assessed for work. If you are 65 you are ok it will carry on the same.. This is only what the DWP said to me, they would only get round to me in 2016 God willing I will still be here. You can try Www.GovUK. or phone them direct, others will be along with more helpful info
Good luck. B.