How to change my contact details on my MS Onedrive account

I have long had an MS account which I use for One Drive and also pay monthly for extra storage. I want to KEEP everything as it is EXCEPT: my login for that account is (still) what was until recently my main email address and I have used it for almost two decades. 

But changes at my ISP which DID run the email service but no longer does that and the ISP has sold the email service to an outfit who are truly awful mean that I am in the process of changing my contact details, i.e. my contact email address, to a new bespoke address - I registered a domain name and use that for email.

NB The new outfit took over in the spring and in taking over the server they LOST all my emails stored on it. I have since been able to track down half of them, but many important emails have been lost for ever. They are a disaster and I have to make serious changes and I very soon hope to ditch that old email address completely.

That brings me to my question: I need to change the login and details of my One Drive account. But I HAVE to keep ALL the files store on One Drive as well as keep the storage space I am renting. Microsoft’s ‘help’ FAQ are worse than useless, confusing and send us round in circles. 

So could someone from Microsoft help me out and advise me? If you want more details, please get in touch. 

PS It does NOT help that when trying to post this, I get an error warning and highlighting in red a sentence but no indication as to what might be wrong.