I am managing a not-for-profit organisation, which amongst other things provides on-site accomodation on a 24/7 basis. Existing job descriptions for staff do not include a provision for ‘on call duties’, i.e. being part of a rota that can provide telephone back-up for staff working alone at nights/weekends, which was forgotten to be included in Job Descriptions. A policy has been agreed now by the board of management, asking staff to participate on the basis of the rota/weekend work already covered in the Job Descriptions. Staff are refusing to do this, claiming that the on-call duties go way beyond what they agree to in their existing contracts/job descriptions.
As we need the system, what is the safest way to proceed? Terminate old contracts and offer new, amended ones?

Thanks for any suggestions
Martin Heuter-Russell