What happens to Disability Premum while PIP is being assesed

I have just got the dreaded letter of doom informing me of PIP changes, as i still get JSA with Disability Premimum what happens to the DP while i am being assessed?

And also if I do not get PIP and I ask for a review will it be backdated 

I am sorry if this has been asked before but this is my first post and hopefully I am allowed one wee mistake thanks a lot in advance