Originally Posted by
Bezalel
If your crank is a Sora it has a 130/74 BCD. Changing only the smallest sprocket without changing the larger sprockets will make you exceed the capacity of your front derailleur.
Theoretically yes. Realistically no. I've substituted a 26T for the OEM 30T granny ring on a bunch of Shimano 7,8 and 9-speed triple cranks and on a Campy 10-speed triple crank. In all cases, the standard front derailleur handled the new combination (52/42/26 or 53/42/26) with no difficulty.
As to fitting a Chain Watcher or Jump Stop? I highly recommend it as cheap insurance.