If your PIP was in payment at the MR rate and you decide to appeal that decision your PIP will stay at that rate. If you withdraw your appeal you will keep the same rate.
The problem arises if your PIP was not in payment before you applied to appeal the decision, in which case I would advise you to seek help from your local Welfare Rights Organisation.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems