As of version 3.1.2, any size tower can be rebuilt using the Rebuild button in the Tiny Tower menu. You will keep your bux, elevator speed, previously earned golden tickets and any cosmetic upgrades but everything else will be reset to the tutorial. You will also earn a golden ticket for every 50 floors you have when you rebuilt.
You can view your past towers by going to the Rebuild menu and tapping "Past Towers". However, once rebuilt, your past tower cannot be recovered. They are only an archive.