If you aren't totally confident in citrus gardening using containers, I would suggest you leave it in the current mix until next spring. We have this privilege living directly in the citrus belt. During the 1st year you learn watering, root temp protection, fert, and pests. More importantly, you learn a baseline behavior for a container citrus. Like what is normal and not for your tree. Then when you repot completely into a new mix in a larger container, you have so many fewer issues to contend with. Container trees are different than in-ground, I always suggest this to my family and friends when they purchase a fresh tree from a retail nursery.

During the first years, there is no need to cut all, or part of vigorous growth, since research has shown that any cut to a young tree reduces root growth. This is due to the equilibrium between the foliage and the root system. A plant is a biological unit. Cuts on a young tree also delays trees coming into bearing.

You probably should have asked this question in the "Citrus" section. That being said, I do know what is most likely causing your problems. Look very closely at all the leaves. Do you see any evidence of leaf miner? (do an internet search if you do not know what that means) Otherwise it is most likely aphids. That white powdery stuff is sugar that has crystallized out of the sweet gooey excretions left behind by aphids. The excretions can attract ants, which is not a good thing, because ants have a sort of mutually beneficial relationship with the aphids (but bad for the tree) where the ants will fight off the beneficial predatory insects that eat aphids and normally keep their numbers in check. All the moisture being sucked out of the young leaves by the pestilent insects results in the leaves being permanently disfigured.

So, you changed the soil, changed the light and temperature all at the same time; and you are surprised that the plant suffers??? Treat it well, and it may recover; citrus are amazingly resilient.