Hi apples,
PIP has not yet been rolled out nationwide for people who are currently on DLA, the process is well behind schedule. In 'non PIp' areas (according to post-code) DLA recipients will be currently reassessed for this benefit rather than being invited to apply for PIP.
With PIP being so new we don't have enough experience to know how older claimants are being treated and we would be foolish to try and guess how future reassessments might be conducted.
Nothing on this board constitutes legal advice - always consult a professional about specific problems