Email – Changing Primary Alias – What happens to former primary alias?

Several months ago, I wanted to make a new primary alias (log-in address) for my Hotmail account (now Outlook), and then delete the former primary alias. But that option was not available. Yesterday, I looked at the section about "create alias," and it seems possible now. Is it true, that I can create a new alias, make it my primary alias (log-in address), and then delete the former primary alias (log-in address)? If so, what happens to the mail sent to my former primary/log-in address, after I've deleted it? I have hesitated to create a whole new account, simply because there is so much involved with doing all that. I would rather, for simplicity, keep my current account and make a new primary/log-in address, delete my former primary/log-in address, and never receive mail from people who send to my former primary (and secondary) aliases. Does the mail sent to the former address go into the junk folder, or does it never arrive at all in my account? What I would like even more is an online service that I can log into with a user name (that is not an email address) and then have email aliases to use within the program. That way, I never risk sending any mail from my log-in/account user name. Would someone who understands this subject please explain it to me (without using many high-tech words)? thanks.