I look at it like this. transition(stretch)/Early flower is when your buds looks like little round hair balls and some new fan leaves may appear, mid flower is when buds start to take shape but with lots of white pistils and maybe a some orange, calyxes start to swell and stack, buds starting to fill out no more new fan leaves, late flower flower is when buds look like the buds we love, colors start to change, pistils start to change color, calyxes are still swelling up but not stacking as much as in mid flower.
You really dont need to change nutrients as long as you're using a good balanced formula. A lot of people (myself included) use the same base nutrients all the way through and use a PK booster at their discretion and according to their own grow style. You can make things as simple
or as complicated as you want and you'll still get great buds. If you look at the charts i linked earlier, they basically tell the story.