I did just exactly that when submitting my wife's old 48 page passport which even had two sets of additional pages. In addition I also had a post-it note on the old passport and another on the application form requesting the larger 48 passport. They just gnored all the notes and signed request and sent back a skinny new 24 page one . We then immediately sent it back for additional pages.

One would think that when they receive an old fat passport for renewal with requests for a larger passport that they'd honor the request but that is not always the case.