I have an 8 speed 2005 Specialized Dolce with Sora shifters, front derailler and tiagra rear derailler. I want to begin to upgrade some of the components, but have no idea where to start. Can I upgrade the shifters and rear derailler without changing the cassette? Could I upgrade the rear derailler to a shimano 105 and keep the front derailler the same?

thanks.