In most cases, the tube is a separate component from the middle case. And unless it has been epoxied in, you can usually unscrew it by sliding a triangular hobby file into the tube so that it is wedged into the tube & then unscrewing the tube. Often, a gen tube (which usually requires a gen crown) can be screwed into the existing hole. Sometimes, the hole may need to be cleared of cement/glue before the new tube can be installed, or you may need to tap the threading to clear it (most of the watch parts supply houses - ofrei.com, julesborel.com, cousinsuk.com, etc) sell taps made for Rolex tubes for less than $40). If you decide to replace the rep tube/crown with another rep tube/crown, peruse ebay for replacements.