You do. Make a bootable USB installer of HS using Boot Buddy or Install Disk Creator, then boot from that to install just the OS. I would advise first cloning your main drive with SuperDuper or Carbon Copy Cloner, rather than migrating everything from TM. Re-download any Mac apps from the App store and non-Mac apps from their sources or the clone. Migrate the files you want from the clone. You can be selective, or do it en masse if you don't want to use this opportunity to clean out unused/unwanted files. If you find any problems with HS, you still have the clone of your previous system to quickly get you back up and running (faster than TM).