Ok, I need to chime in here a bit.
The ratios of a 20-20-20 and 1-1-1 mix will be the same, but the
strength will be different. The numbers refer to the percent strength within the solution, which makes the second one significantly weaker (which is why it is for seedlings).
Usually people overnute, but for reference:
If the plant is undernuted (usually N) it will show it in the lower leaves first. These leaves are diffusely yellow.
Overnuting will also shows up in your 'resevoir' leaves.
The diff is with overnuting, eventually the tips and mid part of the leaf will look kinda crumbly/rusty from accumulated salts. The initial yellowing can resemble the 'interval chlorosis' people talk about with Mg+ deficiency (confusion of overnuting with Mg deficiency is a very common error).
Overnuting is a hazard to a lot of people. We cram the nutes down the plant like Foie Gras.
Marijuana mistakenly gets labeled a 'fruiting' plant when it is flowering one. The nute strength needs are generally different across these two classes.
One great thing is to keep track of your nuting strength with a truncheon, then you start learning what is the 'sweet spot' for you and your water supply (if you want me to elaborate I will).
The best early advice I got was to take it easy and do not jump to conclusions. Take it slow and make a well reasoned change. Do not make multiple changes at once. Find someone you trust and rely on them b/c you can get tons of conflicting advice on forums.
RE: Flowering Nutes: The shift of nutes should be from nitrogen to potassium and esp. phosphorous. Why I don't get is that a lot inexplicably make the switch the nanosecond they go to 12/12.
Personally I think the timing of when you go to veg is less significant (as long as about 30dy) than how you transition the nutes.
I hold off on flowering mixes until I have sexed the plants. That's on avg 11-14 days into flower!
The end is the plant has it's stores of N for the 'big push' into bud production. I call it the foundation you lay to build the big azz house!