For anyone who didn't get it, in Manage bookmarks there is a button named File on the toolbar which displays a menu including Open which will open a bookmarks.adr file as your active file (and obviously close whichever file you were using before). The same menu can also be found as a submenu when you right-click a bookmark in the panel, if you prefer. Though you could also change it in opera:config directly, but that may result in the file being overwritten (if it exists).
An alternative approach to this particular question - on that same menu is a Save command, which allows you to save your bookmarks as a new file, but doesn't remove the old file. (And yes, Save does make the new file active.)