Rewiring speakers to change the effective impedance of a cabinet: here's a link that covers it, and does a better job than I'd probably do.
http://www.bright.net/~robertso/speakers/spkrcon.html
Note that you can't always get what you want: if you have a cab with 4 16ohm speakers in it, you can get either 4, 16, or 64 ohms as the final impedance (parallel, series-parallel 2x2, or series). Assuming, of course, that you don't want to leave some drivers unconnected....
Really dealing with impedances. That's a good can of worms, and gets into plastic-pocket-protector-and-taped-glasses territory pretty quickly. I was treating "impedance" and "resistance" as equivalent for simplicity, and nothing could be further from the truth: especially, as it turns out, with guitar speakers...
The fact is that the AC impedance of a raw speaker driver varies all over the freakin' map, depending on the frequency of the drive signal and the nature of the cabinet. Drivers are rated with a "nominal impedance" value, say 8ohms. But if you actually do a sweep measurement, you might find that the actual impedance varies from 2ohms at 20Hz to 40ohms at 10kHz, with lumps and dips all over. What does this mean to us as musicians, who need speakers for our performance amps? Less than you'd think, turns out.
Usually, with tube amps, they'd prefer to be loaded with a lower impedance than a higher impedance. Run a tube amp wide open with no speaker load connected (an infinitely high output impedance), and watch it fry from flyback-effect overvoltage: tube sockets arc, nasty things happen. But if you overload a tube amp (drive a lower impedance than rated), you'll usually just get a hot output transformer. Tube amps can usually drive down to ~2 ohms nominal load without too much strain: they will just be very inefficient, and run very hot, if run at lower than their rated impedance. The hotter the output tranformer gets, the less efficient it is, which reduces the power transfer into the load (a negative temperature coefficient). Usually the system achieves a safe (but smelly) operating point. It won't do this forever, obviously, but it won't blow up instantly.
Transistor amps are just the opposite: run a transistor amp rated for a 4ohm nominal load wide-open at 2ohms, and watch it go into thermal runaway (which typically lets the Magic Smoke out). The output transistors conduct _more_ current the hotter they get (a positive temperature coefficient), so the transition from "toasty" to "Pfft!" is usually vigorous, instantaneous, and even _more_ smelly... But take the load off of it altogether, and it will usually not care too much. Different beasts.
Anyway, I'm quite convinced that this is one reason that tube bass amps work so much better in performance. Let's say you're driving 1 15" driver in a bass reflex cabinet, and the cabinet's porting is screwed up so that the resonant frequency of the driver/cabinet/port system happens to exactly correspond to the minimum impedance point of the driver in free air. The result of the cabinet resonance will be to drive the speaker's impedance down even _lower_... Let's say that that's 1.8 ohms at 40hz, just for amusement. With a tube amp driven hard, your sound will get just a little graunchy there- you'll get some extra even harmonics as the output transformer saturates, and you'll get that wonderful smell of warm Tolex. With a transistor amp, you'll get generalized discomfort, as the amp's output stage tries to stay alive: odd-order harmonics, power rail sag, thermal protection kicking in, yadda yadda. Yucko. It's a different form of distortion, and one that I don't care for as much. Death metal folks might disagree, though... (;-)
Basic refresher: you can go lower impedance with a tube amp, within reason. You can go higher impedance with a transistor amp, within reason. Both are most comfortable when driving their rated load, both get uncomfortable when driving not-well-behaved loads: they just get uncomfortable on opposite ends of the scale... When in doubt, believe the nominal load ratings, and match them as closely as you can.
since you are on a roll now....