Uh-oh - now that I think about, it's planted near a rose bush that I did give slow-release food in the soil near the roses' roots earlier this spring. If I dig the tree up & pot it in unfert. soil, could it go back to variegated?

I don't think the fertilizer would affect the color. Variegated foliage sometimes pops up in regular plants, so I suppose it can revert in plants as well. I'm seeing some strange things after this freeze. All you can do is wait it out.

My only other thought would be if it was grafted and all the graft growth (which would be variegated) was killed in the frost or over the winter, and now the root stock was taking over.

I'm wondering if it's an asahi zuru instead of oridono nishiki since some of the leaves are beginning to show variegation now. On the UBC website people describe their asahi zurus as starting out green-leafed in spring. If all the graft growth died, almost the whole plant would be dead since the graft is low.

Update - the tree is a lovely variegated Oridono Nishiki for the past 8 years.

Awesome! We would love to see a photo of it!

Variegated Japanese maples are very prone to reversion. It is likely nothing more than this.