(b) In exceptional circumstances, a court may modify a parenting plan if it finds that the plan is not working as contemplated and in some specific way is manifestly harmful to the child, even if a substantial change of circumstances has not occurred.
(c) Unless the parents have agreed otherwise, the following circumstances do not justify a significant modification of a parenting plan except where harm to the child is shown:
(1) Circumstances resulting in an involuntary loss of income, by loss of employment or otherwise, affecting the parent's economic status;
(2) A parent's remarriage or cohabitation; and
(3) Choice of reasonable caretaking arrangements for the child by a legal parent, including the child's placement in day care.
(d) For purposes of subsection (a) of this section, the occurrence or worsening of a limiting factor, as defined in subsection (a), section 9-209, after a parenting plan has been ordered by the court, constitutes a substantial change of circumstances and measures shall be ordered pursuant to section 9-209 to protect the child or the child's parent.