Or if you were playing the aforementioned Greensleeves--Kevin--Songs that start on an Am and move to an E7 are not that uncommon--Johnny I Hardly Knew You come to mind immediately--or are you such a purist that you forsake diatonic melodies?

Damn you Don!;-) You owe everyone an apology for posting all that!! You have hijacked the ship and taken it irretrevably into the nether realms--And I am only kind of kidding--I understood what you were saying, but those that don't know what you are talking about already will choke on it all(as Dave4Guild has pointed out)--Even still, I am biting my tongue, knowing that my nitpicky additions will make an even worse muck of this than it is, and, sadly, always seems doomed to be--

The only conclusion I can come to is that it isn't possible to provide good answers to music theory questions informally--in order to come to a real understanding you have to do a detailed analysis of specific pieces of music, and you can't gloss over things. because the gaps always get thrown back at you--

Still, all we can do is try--

And, to that goal, my little contribution is that the shift that Don is talking about, which becomes possible with the addition of the leading tone, is the shift from tonic harmony to dominant harmony, and it is the use of this shift that makes a melody diatonic instead of modal--