Changes in smell will just be down to the overall acidity, which reduces for a time at each feed. Time to increase acidity again will depend on the volume of old starter, activity of old starter, volume of feed and temperature. I let my starter just tick over for long periods, sitting on the counter, with a week or two between feeds and this keeps it quite acidic and great for pancakes. I then ramp it up with daily feeds for a few days when I want to use it for bread.
When I first started to make bread, being too frugal to keep buying dried yeast, I was simply keeping about 20% of my risen yeasted dough back to use in the next loaf. After a few weeks it had morphed from a yeast dough to sourdough. The lactobacilli outcompete any bought-in yeast cultures, especially when the pH drops and it becomes more acidic.
I reckon that unless you are purist, by far the easiest way into sourdough bread-making is to make a yeasted loaf, keep some of the dough back, then just feed it as you would a sourdough culture. After a very short time, it will be indistinguishable from any other local sourdough culture.