I think you misunderstood what happened. Cltr+Backspace eliminate the innermost environment around the cursor. In your case the second-level itemize
environment, making seems that you have “unindented” the items all at once. There is no easy builtin way to change the itemization level of a specific item. See the discussion here, for example: Itemized list editing is a bit counterintuitive? .
Note also that removing the innermost environment should be done with care, as sometimes the innermost environment is not really shown in the page, e.g. when doing beamer slides and you might accidentally erase a whole lot of document structure.