That is absolutely true!! Once you age up a newborn to an infant, you can select the expansion pack (E13) on that new infant and SOMETIMES you can select the traits/quirks because none of them are already grayed out. But if there are any grayed out options, the game selected those.
If you change them before they are discovered, will they show up? I don't know for certain about that, YET!! I have new infant I am testing that on.
But the 2 in the pictures, those I changed after they were "discovered". The game removed the PickyEater from both toddlers from the panel, and as of yet, the only traits/quirks showing in the panels are the ones the game selected - none of the ones I selected are showing even after they had been discovered.
To answer your question: Yes, I had both doing the new quirks - I want to say that in the notification box it acknowledged the "changed quirk" (Loveswater) and the animation of the discovery happened, but the Loveswater trait/quirk is still not showing in the panel. I don't recall a notification for the "LittleSinger" quirk but I know I had her singing! That was adorable!!
Again, the only one's not showing in the panel are the ones that I changed after they were discovered.
I changed the undiscovered quirks in one of my new infants late last night - just changed the grayed out quirks to ones I want to try, now I play , and wait for them to be discovered to see if those will show up in the panel or not. I don't have a lot of time to play today but I will post what I discover about the changes I made in the EP13 cheat as soon as possible.