Upgrading components: 9-Speed to 10-Speed, going to 105 or Ultegra (Shimano)
 

I currently have a 9-speed road bike using Sora and Tiagra(rear) derailleurs. If I wanted to move to a 105 or Ultegra, they are meant for 10-speed, would I have to change out other parts like the crank or cassette to accommodate?

You would have to at least get a new right shifter, cassette, chain, and a cable.

Here's my 9 speed/10 speed mismatch: SRAM 9 speed chain, Tiagra Shifters, Tiagra Crankset, Tiagra Cassette, Ultegra 6600 FD, Ultegra 6600 RD. This is easy to set up and my shifting is MUCH better than when I was using the Sora FD and Tiagra RD.

Also, I have done a good amount of reading on the subject, and the Tiagra Crankset should work fine when I decide to upgrade everything else to10 speed. This setup is working so well that I don't know when that will be.

So, I would have to change to a 10-speed?

Could I still use the existing 9-speed cassette, and just forgo the 10th (28th, 29th, 30th) speeds?

you can upgrade your components to 105 or ultegra (older versions {ebay}) and keep the 9 speed setup, this way you dont have to buy the shifter, chain rings, chain or cables. if you wanted a different gearing combination you can get a 9 speed cassette w/higher or lower cog limits. a 10 speed setup may have the same number of teeth on the largest and smallest cog just the steps between them are lower, it wont translate into more speed necessarily.

I'm finally making the switch to 10-speed. I'll have the build installed tomorrow. I'm changing the entire group.

I don't get why you would upgrade without changing to 10 speed. That's the primary reason to get 105/Ultegra.

No, you can not use the 9 speed cassette. The spacing is different. You'll need to change the cassette and the chain if you go to 10 speed. You can continue to use your cranks and chainwheels and derailers.