Made to change by insurance provider

My wife is a retired teacher who has health insurance through the state employee retiree program. She has been asked to work a 120 day interim for a maternity leave teacher. She is being told by the HR director that at 90 days she must drop her retirement insurance and take the school system insurance. It is similar plans from the same provider but the HR director is very concerned about disruption of coverage and is discouraging my wife and the principal from doing this. It is in the best interest of the student to have an experience teacher for this time. Why can she not just keep the insurance she has. The HR director says the provision of Obamacare mandates the at 90 day every eligible employee get the system insurance whether they want it or not.