Tiagra is 9 speed. So you will need to change out the cassette and chain Maybe even the rear derailleur. Not 100% sure if the Sora 8 speed rear derailleur will work properly with the 9 speed shifters. According to Shimano, the Sora RD-3300 rear derailleur is 7/8 speed compatible. The new Sora RD3400 is 9 speed now.