Is there any way that I can "ungraft" a Euphorbia Lactea Crest from a dead cactus stem and replant it without killing it?
As long as the Euphorbia Lactea Crest graft is still healthy, you can definitely take it out and re-plant it. However, you have to take good care of the graft, and preferable re-plant it immediately after un-grafting it.
You just cut it out and replant it. They sell stuff to help it grow. Make sure there is good drainage so it doesn't rot. Hopefully that is plant that roots easily. It doesn't always work.