Merging two microsoft accounts

Hi, in current tenant we are using abc.onmicrosoft.com as account name for all users, so all our Microsoft products (office, teams etc) need user to login with user(AT)abc.onmicrosft.com

Now we would like to use our company domain company.com, where user then can login use user(AT)company.com

Question is in our company some already have their user(AT)company.com registered as a microsoft account. So when we switched to our custom domain, what would happen to their existing individual account with user(AT)company.com? Would the two merge or they don't have access to their old account anymore?

Thanks for clarifying.