What is the name for this operation?
Effectively, take a range and adjust its center point to 0 on a number line.
In the case of the example above, I'm specifically looking for the name or operation which takes a number range of -28 to +72, and adjusts it back to -50 to +50.
I should also say, I'm looking for the terminology that hopefully doesn't require a math background. If the official terminology of the process is a pretty technical word, than I would love to see a simpler explanation that would suffice. Something a freshman in highschool would understand. Or even a middle schooler.

To make it understandable to a middle-schooler, you might say that you slide (or move) the whole interval so that its mid-point slides (or moves) to zero.
As have already been mentioned, the technical term is translation, which children might associate with translating between languages. They are absolutely right to make the association, since a word in one language is coupled to a word in another language, just as a point in the interval [-28,72] is coupled to a point in the interval [-50,50].