I started again to receive warnings and threatening messages regarding a possible Yahoo Upgrade. If this a real info or just another Spam?

Answer 1

All that is a definite scam by what you asked about. Scam messages that request you to "upgrade" your email account must be ignored and deleted immediately. Never click on the links or attachments and give your personal information to the scammers or you can risk your account being hacked. Remember Yahoo would never send you unsolicited emails asking for your personal information.

Answer 2

Probably spam, but without knowing the exact content of the message or the email address of the sender, we can only guess. 

Answer 3

We have no way of knowing.

Answer 4

That is a phishing scam. Ignore and delete it. This is how to identify legitimate Yahoo websites, requests, and communications: https://help.yahoo.com/kb/SLN2091.html