Repack a CBR comic as a CBZ, with every page copied as it is - here in this tab, with nothing uploaded.
CBZ opens in more apps than CBR. The archive is unpacked and repacked in your browser; the images themselves are never touched.
No. Every file is copied byte for byte; only the archive around them changes from RAR to zip.
No. The file is opened by your browser, in this tab, and the new file is made here too. Nothing is sent to a server, so there is no copy of your book anywhere else.