To change the email address registered on your Magix Account go to your account page, log in (clearly using the old email) go to User Data. Towards the bottom of the page that opens click on "Change Details". Make sure you save the changes.
So, the problem you have is two-fold, if I've understood your latest post. Not just that your email address has changed but that you have forgotten your password for your Magix login!
In the circumstances, I think your only solution is to email infoservice@magix.net, explain the problem, giving both your old and current email addresses, and asking for their help. Hopefully, they will be able to resolve the problem for you but, given that all Magix employees are currently working from home, you may not receive a reply for some days.