Peter, I don't think a key change is what you're dealing with. All of the chords in your example are legitimately used in the key of G. As to how to know which chords to use, yes, hunt around--but hunt with a map.

I'm no expert on this, but I have read that the melody is rooted in the chord structure. This may take the form of our subconsciously being attracted to melody lines which fit in with certain chord patterns--we may think of the melody first, but the chords are there, just waiting to be let out.

Chords commonly seen in popular music, in a rough overall descending order of frequency, would be I, V, IV, V7, VIm, IIm, IIIm, I7, II7, III7, VI7, IV7, IVm, VIIb, IIIb, VIb...there's more, but the point is, the I, IV, & V7 chords are found more often than the VIm, IIIm, & IIm chords, which are found more often than the VIIb, IIIb, & VIb chords. It helps to have these possibilities and probabilities in mind when looking for the next chord.

One thing that can really guide you is the circle of fifths. Click here for a nice web page diagram with helpful notes. This is probably the "powerful chordal structure" you read about. Note how the IV & V are on either side of the I. If you jump from I to III and come back to I one step at a time, there's the jazz sequence you mentioned. In the other direction from I, you'll find the less common VIIb, IIIb, & VIb. The chord directly across from I is virtually never used.

It is very rare to have a song that spans more than six adjacent points on the cirlce. For example, if it has a III, it's a pretty sure bet it won't have a VIIb. Sometimes it works through its range on the circle a step or two at a time, sometimes it jumps around haphazardly.

As far as actually changing keys, there's no single way. Take "Lemon Tree" a la Peter, Paul, and Mary. It goes from D to G to C. Other songs are often done with each verse a half step up the preceding one; or only the last verse going up, by a whole step. Some change key in the chorus, going to the IV or V key, and come back to the original key for each verse. Sometimes a key change is signaled by a 7th chord, but not always.

Well, that's probably more than I actually know about it (BG). I hope that addressed what you were looking for.