Scheduled release for activities

Hi. I have created a Course with several modules. However, I have activities which can only be released to the audience at different set date and time. Is there a way I can go about doing this in an existing (running) course? The issue I am getting right now is that when I change the content of the course by adding activities to the modules, the student's previous achievements will be deleted -- > "The contents of that training has changed. If you start again that training, your previous achievements for it will be deleted. Do you want to continue? Yes No.