Bottom fan leaves turning Yellow on week 8... Emergency, or all good ?

Looks like u still got a few weeks left. But as long as yellowing is limited you'll be fine. Autos tend to like to shed a few leaves around week 3-4 of flower than followed by an explosion in flower development week 4-5 of flowering. Usually autos will be full mature after 6 weeks of flower. Some chop after 5 but imo too early. But of course strain dependent but I've grown many autos 5 weeks minimum.

Mention : Plant on Week 8, but in flowering since 3 weeks. (it's an auto). Only bottom fan leaves turning yellow, all the rest is perfectly green.

The yellowing of plant leaves is simply due to a nitrogen (N) shortage which is a perfectly natural phenomenon as the plants nears the end of its life cycle. You are in the eighth week of flower, so you are likely close to harvest time depending on your strain. Most of today’s commercial strains flower for about eight or nine weeks. During the latter weeks of flower, the plants have a physiological shift that requires less nitrogen (which is more important at the start of flowering) and more phosphorous (P) and potassium (K). Many nutrient lines adjust their programs to account for this change, and as less nitrogen is present in the feedings the plant’s leaves begin to turn from green to yellow. However, sometimes the nutrient program is a little too deficient in N and it sounds like you may have a bit of this issue here as the tiny leaves of your buds should generally not be turning yellow. One or two here and there is OK, but if the yellowing is rampant on your buds a boost of N could help. However, if you are just days away from harvest, then you should be in your flushing stage (feeding with pure water only) and I would not recommend adding anything further. If anything, you may want to expedite your harvest and cut down a few days early if you think the buds are really suffering. In general, around week six or seven (or two weeks before harvest to be more precise), most growers begin flushing their gardens. Flushing is another way of saying watering or feeding with only pure, distilled water. No nutrients are added during this time. This helps the plant rid itself of excess nutrients and additives that build up over time. It also helps to create a cleaner, healthier smoke for you after harvest. Well-flushed buds will dry and cure easier and burn to a clean, white ash when smoked, making all your hard work that much more enjoyable in the end!