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You shouldn't worry about them being rootbound until you see lots of roots trying to get out and/or stunted growth. I don't think you can transplant too early. The only bad thing that could happen is ripping the roots if there are not enough of them to hold the soil together. Still, if you're careful that can be avoided by moving the soil, not the plant. In other words, don't pull it out by the stem. Get your hands dirty!
dont be suprised if you dont see much growth for the next few days, after a transplant your roots will be growing longer seeking the bottom of your new container.
once the roots hit bottom your plant will start growing up top faster, good luck!!!
My experience with a couple of root bound plants I had was that the bottom leaves would start to turn yellow. They were in 4 1/2 inch containers and a party cup and started showing signs between 2 and 3 weeks. As soon as I got them out of their containers and could see the root mass I knew right away the problem was root bound. After transplanting they did just fine. So since then I transplant at the 2 week mark straight into a 4 or 5 gal container. I would say you're not going to hurt anything by transplanting if it makes you more comfortable. I've never used rockwool cubes but if you're seeing roots or a lot of them then I would transplant.
Plants can't really get root bound in rockwool...but what does happen, is they start getting too long, and get tangled up with the roots in the cubes next to them, and you end up breaking a lot when you break them apart to transplant--
You did well by transplanting when you seen them--
Now that you have them in containers, I presume it is soil??
In a couple of weeks, before you water, turn the cup upside down, supporting the soil, gently lift the cup off of the soil, and look at how the roots are doing-- If you see them throughout the whole soil, they are ready for transplant...if the roots are circling on the very bottom...that is "Root Bound"--