An eligible employee may elect, or an employer may require the employee, to substitute any of the accrued paid vacation leave, personal leave, or medical or sick leave of the employee for leave provided under subparagraph (C) or (D) of subsection (a)(1) for any part of the 12-week period of such leave under such subsection, except that nothing in this subchapter shall require an employer to provide paid sick leave or paid medical leave in any situation in which such employer would not normally provide any such paid leave. An eligible employee may elect, or an employer may require the employee, to substitute any of the accrued paid vacation leave, personal leave, family leave, or medical or sick leave of the employee for leave provided under subsection (a)(3) for any part of the 26-week period of such leave under such subsection, except that nothing in this subchapter requires an employer to provide paid sick leave or paid medical leave in any situation in which the employer would not normally provide any such paid leave.