Do I need to keep my old antivirus to transfer from Dell to MacBook?

On Macs, installing anti-virus and so-called "cleaning" apps practically guarantees you will have performance and stability problems. Don't make fear marketing pay off from someone.

Still, everyone get bogus mail, texts and alerts. Your best protection against those lies between your ears. Bookmark this Apple article for reference if you see a "you are infected!" or similar suspicious message:

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Also remember this: NO ONE, not even Apple, can "see" what is on your computer, phone, or tablet unless you give specific permission for remote access.

To make sure you are contacting Apple for support, get the contact information ONLY from Apple.com, not from some "look-alike" support site that pops up in a search engine. The first few hit are almost always paid placement and NOT Apple. Too often they are fraudsters.

No, PC's can be infected by a virus. A Mac cannot. Save your money.