Hi Benefitshelp
Welcome to the forum, you might want to have a look at the following FAQ which explains where everything is.
Welcome to Benefits and Work .
You will not have any overpayment as DWP extended your award until a new decision has been made, your award will only stop once a decision has been made that you are not entitled to the benefit.
We have a very substantial guide on the PIP process and it will walk you through every step. The review form is slightly different from original PIP claims form but the questions are the same, the guide below does go through the review form see page 102.
www.benefitsandwork.co.uk/help-for-claimants/pip Gary