medicare available for disabled person under age 65

Daughter received Medicare card effective 7-01-11 when she will turn 20 years old.  We have
a HMO presently that is the primary insurance and she is in MediCal as well.  She is permanently
disabled and received SSDI benefits.  All of her prescriptions are covered by a prescription plan
under the HMO.  Will MediCal force her to take Part D of Medicare even tho she has coverage with
the HMO?  How can we get around this and keep the HMO paying the prescriptions.  MediCal presently
only pays the co-pmts generated by the HMO after they have paid the primary costs.  Seems as tho
keeping the HMO prescription plan would be more cost effective for MediCal but it is our understanding
MediCal computer system will try and change her to Part D once she starts Medicare.