2. Right, but consider taking a snapshot of the PV2010 state before editing in PV2012 - not necessary, but may be comforting...
3. Upgrading Lr3 catalog creates a *new* Lr4-compatible catalog. Your original Lr3 catalog will not be altered, but will take the role of backup, since you'll probably never need or want it again.
No photos will look any different after upgrading your catalog, until you edit them.
Most people don't convert photos from PV2010 to PV2012 unless they are planning to re-edit them immediately.
In addition to Rob's comments you should also note that:
1. The 'blacks' conversion from PV2010 to PV2012 can change quite significantly on some images (i.e. those where the blacks slider had been pushed up above 10). This can result in image lookng quite different after conversion, but is usually fairly easy to fix by readjusting the PV2012 black slider.
2. The hue of natural yellows that tend towards the highlight end of the scale will tend to shift a little. Fixing this isn't that difficult and in some cases you may prefer the new rendering.
Just to add to what Rob and Ian said: if you convert from PV2010 to PV2012, the result usually looks unchanged. However, sometimes the conversion may result in a slight or occasionally significant change in the appearance. In my experience, it seems to be images with major changes in basic panel or tone curve that are likely to change in appearance.
My conclusion: let LR4 convert your entire catalog to its new format, which means it creates a new catalog (saving your old LR3 catalog) but with all images still in PV2010. But don't convert images to PV2012 unless you're going to do further edits. Converting on its own won't improve the picture, and may change it (which means make it worse, assuming you'd edited it to how it looked "best").
With exception of blacks, the other basic PV2010 adjustments should convert with little or no change. Changes resulting from tone curve adjustments ( i.e. point curve) are down to a bug in Lr4.0, which was fixed in Lr4.1.