I have had this snake plant for over 6 years now and just recently repotted it with a mix of soil, orchid mix and perlite as I noticed it was going yellow. I'm not sure if half of it wasn't getting enough light, the roots didn't have enough room due to being in the same space for a while or if I messed up Watering it. Since I repotted it over a month ago, I have watered about twice and took the inside pot out so the water goes all the way through now(which I never did before oops). What can I do with the old/yellow leaves to help them? Some of them are very yellow at the top and one of them is completely bright yellow.
As a general indication, some nutrients are more "mobile" and they can be "recycled" from the old leaves (which will drop at some time) right into the new leaves. This means that nutrient deficiency will show-up on the old leaves first. "Immobile" nutrients are the ones which cannot be recycled from the old leaves and fed into the new ones. Deficiency will appear first in the new leaves. (Iron, Manganese, Calcium). These nutrients will be recycled only when the leaves fall and decay. Source here: University of Maryland Extension - Nutrient Deficiency.
The plant may have have suffered from few things before being repotted:
- Salt buildup - As you said, the plant was not watered until the water dripped from the drainage hole, leaving some salt build-up, which may have raised alkalinity.
- Rootbound - The roots may have done one (or more) full circles around the rootball. This is not recommended, and the best is to untangle the roots. With succulents, the less you touch the roots, the better, but if the roots got so big, just untangle them. Move the plant into a greater pot. Try to straighten as much of the roots as possible (Make them point outwards) and only trim the ones which exceed the inner periphery. (Keep as much of the root as possible, but let it sit straight and pointing outwards and downwards)
- Lack of nutrients - Add slow-release pellets to the mix, or feed with a weak fertilizer solution.
What to expect - I don't think you will see a change in the old leaves. Focus on having a healthy growth at the center, where the new leaves come-out. With time, the old leaves will dry-up as the plant recycles available nutrients from them. You will be able to pluck them out easily when dry, leaving the younger healthy growth.