rootbound? ok for me?

I have a plant growing inside of a red party cup I am new to growing and had no idea how roots actually grew ... as my plant seemed to take off and do really well I noticed roots were reaching the bottom of the cup I planted it into something about twice the size but I have a few others that now have reached the bottoms of the cups also ... I need them to be small but dont want them to die if left alone will I be able to flower them and have them produce buds just be smaller with less yield?

and while im typing this when is the earliest I can start them on 12-12 they arnt very big which is good and have about 5-6 nodes right now still vegging

you can start flowering at any time now,,,, make sure you have more than enough room for roots to grow or you will run into a problem while it budding and it will seriously affect the outcome,, potency, size, taste, and so on...