I just went out and checked on him this morning, and it now has red to black tips at the very end of the point of all the leaves. I'm getting very nervous. :(

Ideally, transplanting or potting up should occur while the tree is dormant - doing so while the tree is in leaf in spring can cause transplant shock. I'd also be concerned about the "much larger container". Generally, potting up should be limited to just one or two sizes larger. Too large a container can hold excessive moisture in the soil mass too long, creating drainage issues. What kind of potting soil did you use and just how big a pot is it now in? Sango Kaku can take a great deal of sun as well, especially here in the PNW.

"Over-potting" is a term that arises from a lack of basic knowledge about the 3-way relationship between plant material/ soil composition/ container size. It's often parroted that you should only move up 1 size in containers when "potting-up". The reasoning is that the soil will remain wet too long and cause root rot issues. Of course, that's true if you're growing in a heavy, inappropriate soil, but it can be remedied by changing the soil to one that drains freely

Here is a link that might be useful: What is so bad about 'overpotting'? - Container Gardening Forum - GardenWeb