first person
First person and third person limited are essentially interchangable, and switching from one to the other really is almost always as simple as changing pronouns and proper nouns. I've done it many times. If it isn't almost always interchangable you've messed up on third person limited POV.
Fist person has a simple rule. You can only write down what the protagonist sees, hears, feels, smells, tastes, or knows. Third person limited has exactly the same rule. You can only write what the POV character in a given scene sees, hears, feels, smells, tastes, or knows.
The only real difference between first person and third person limited is that in third person limited you can use more than one viewpoint character, but you can't use more than one in the same scene. If you do it's called head-hopping and it's a bad thing. Because third person limioted allows only one POV character per scene, switching from first to third, or tird to first is a snap, and takes almost no practice to do perfectly.
All the third person limited I write, even multiple viewpoint novels, begins as first person all the way through.
Third person limited is nothing at all like omniscient. There's almost zero similarity. Third person limited, in fact, is an offshoot of first person, and was developed because it is written almost exactly like first person, but allows just a bit of distance.
It doesn't matter in the least whether the POV in one chapter is the protagonist, and the one in the next the antagonist. This changes nothing in how it's written. The same POV rules apply to both characters. You only have to decide whether you want to write "I" or "He," or "Insert Proper Noun."
And, of course, there are many writers who write as Ben Bova does, which is one character from page one to page last. Multiple viewpoint is not automatic, and is often done simply because the writer doesn't know how to show what's happening somewhere else without having a viewpoint character be there. There are just as many single viewpoint third person limited novels as there are third person multiple viewpoint novels.
In fact, first person and third person limited are so similar that if you're writing first person correctly, there will be long stretches of the novel that don't even need coverting in any way.
"The plane seemed to tremble in the air, then dropped abruply, slamming into the ground and disintegrating into fire and smoke and strewn bodies."
Now, is that first person or third person limited? The answer is that it's both or either. If first person is written correctly, just about all the narration actually will be third person limited.
The thing is, you also have to stay with the "I" in third person limited, you just have more than one "I," and you change the "I" to "he" or "she."
But first person and third person limited are the same thing, only with distance added for the comfort of the reader,
For that matter, you aren't limited to one POV character in first person, either. There's no rule at all that says you can't have more than one first person character in a novel, and it's been done more than once.