yes. you can upgrade different bits at a time. the problem with sora is that it's an 8-speed group, and if you want to upgrade the cassette to a 9-speed, you'll need a 9-speed rear derailleur and new shifters. if all you want to do is change the to a different 8-speed cog spread, you can do that, but you'll probably have to get a new chain as well.

as for the chainrings... there are a couple of ways to do this. the first is just to get new rings [say, a 50t to replace the 53t], unbole the old ones and bolt on the new ones. it's not a terribly involved process, but it can take some patience for a first-timer.

you can also change the whole crankset. if you switch to a splined crankset [say, 105 and above], then you'll have to change the bottom bracket as well. the good news is that a 105 bb costs about $20.