Well, it may have been just restin' after all! (embarrassed rictus-like grin in McGrath's direction)

Hapiness, all you can do is wait until you can definitely identify the branch as being one or the other. You may be able to do so by the leaves. If not, you'll have to wait & see if it brings forth plum blossom.
In either case, I think you'd be better off getting a new young tree. Even if the new growth on the old one IS plum, it may take a long time to recover.