Morgan took over from the 77 year old after he announced plans to leave his nightly show last summer (10).
The British journalist dominated headlines ahead of the January (11) relaunch - but his interviews have failed to win over as many viewers as King's chat show.
King praised Morgan for being a "fine broadcaster" last week (end20Feb11), but slammed him for talking up the format too much.
He said, "One of the problems they did was to oversell it - they said he was going to be dangerous, he was going to be water-cooler talk, it was 'Wait till you see me, I'm different.' Well, he's good - but he's not that dangerous."
The two men went head-to-head on Wednesday as King returned to CNN to be interviewed by Morgan, who confronted him about the remarks.
But King stood by his statement, telling Morgan, "I haven't seen you dangerous yet. I said you're capable. This is what I think. Supposing we're going to do four specials for CNN... first one's in April, we're still working on them. Supposing I said, 'This first special will be the greatest special in the history of American television - the best - and dangerous.' It can't live up to that... Why can't you just say, 'Piers Morgan: I'm coming, watch me.' 'Piers Morgan starts next Monday.' What's wrong with that?"
Morgan fought back, insisting he had no option but to boldly promote the show: "I suppose I've always oversold myself. I quite like doing that, it's quite funny... What was I supposed to (do)? We couldn't come in and undersell it, I'm following a legend. You can't follow (Frank) Sinatra in Vegas and say, 'By the way, I'm not very good and this is going to be useless.'"
And the former America's Got Talent judge was full of praise for King, telling the star he hopes he will one day live up to his "dangerous" request.
Morgan adds, "Every day I do this my admiration for what you've achieved grows, because I've done 22 shows. I feel like I've been in a war zone! And you did 7,000 shows in 25 years... Just the sheer stamina that you showed is awesome.
"I carry a great sense of responsibility to live up to the legacy you left for me. I want you to watch the show and feel that it gets better and feel that I'm dangerous."