Upgrade Shimano Sora shifters to 105 I want to upgrade my 3x9 Sora shifters to 3x10 105s.  However, looking at the Shimano Compatibility Chart, it appears to indicate that I'll have to replace my front and rear derailleurs in addition to the cassette, which I knew I'd have to replace.  But I'm not sure I'm reading that chart correctly.

The bike currently has a Shimano Sora bottom pull front derailleur and a Shimano Tiagra rear derailleur.  Can I get by with just replacing the cassette?
I posed a similar question (or at least a question which raises similar issues) on here a few months ago: Moving tfrom Shimano 9- to Shimano 10- speed

The issue is not the different Shimano ranges per se, its going from 9 to 10 speed.

You can imagine that the 10-speed shifters will be expecting a 10-speed cassette at the other end of the wire - these things are indexed after all.

The other complication in going from a 9- to 10-speed cassette is the chain. You'd need a new, slimmer one. And unfortunately this means a new real derailleur too. Strictly speaking, this again knocks on to the front derailleur. And the chainrings. But as regards the front setup, I was advised that I should be able to get by with my existing setup.

Unless you specifically want to go to 10-speed, you may have some luck doing what I did - jump over 105 altogether and head for the old Ultegra 6500 range.....which was 9-speed. I suspect if you could find shifters from this range, it would do the trick. Alternatively you could go not quite as far and upgrade to Tiagra - I believe the current Tiagra range is now 10-speed, but its not so long ago that it was 9-speed. (The bike I raised my question about was a 2011 model.) Obviously Ultegra will give you better quality but you'll still find Tiagra shifters a noticeable step up from Soras. 

If you really do want to go to a 10-speed 105 setup, well I think I costed it as something like GBP400 or more...not a lot less than buying the whole 105 groupset (which can be bought as a single product). But I seem to remember the biggest single cost was the shifters, so take that as an indication.