upgrading components... can I...

[johnnyh]

can I swap out Sora 8 spd shifters, 8 speed cassette and rear derailleur

for Tiagra 9spd shifters/brakes, 9 spd cassette and Tiagra short cage rear

keeping the existing chain rings/cranks and front mech from the 8 speed set up?

 

[andrew_s]

Yes

You could keep the rear mech as well, unless it's knackered.

 

[johnnyh]

cool, a little project on the horizon then

 

[Gerry Attrick]

You would need to use a nine speed chain too.

 

[RecordAceFromNew]

You would need to use a nine speed chain too.

+1, and also whether a new short cage Tiagra (or indeed the existing rear mech) will be problem free is dependent on what the chainset's and new cassette's ranges are.