You will also need cassette to match the shifters because the cogs have closer spacing, see
http://www.sheldonbrown.com/k7.html. I would advise you to upgrade to 105 shifters rather than Tiagra. The Sora is a simple mechanism well made, whereas the Tiagra is a complicated mechanism cheaply made - you need 105 if you want good functional quality. However if you want to keep your costs down I will buy your SORA shifters and cassette. PM me if you are interested.