Our company has an office365 plan with onmicrosoftcom

Hi Everyone,
Our company has an office365 plan with .onmicrosoft.com domain and we are planning to migrate to a real domain name. Most of our employees use there .onmicrosoft.com email address to log in to office 365 services, like Power BI, Azure ML, etc. 

 Now, what happens to there login to Power BI or Azure ML? Can they still log in to those services with there old .onmicrosoft.com account or they have to create a new account with the new email id?

they have to create a new account with the new email id? and  do we need  a cutover to do this activity 

help me with steps involved for all users or steps involved in it ????

Thanks