Can I upgrade cassette without replacing rear derailleur?
The derailleurs are usually matched to the size of the cassette/chainrings.  

the cage length of the rear derailleur is matched to the largest cog.  that is why you see mountain bikes with a 34 tooth in back with long cage derailleurs and road racing bikes with the little derailleurs matched to the corn cob size cassettes.

The curvature of the front derailleur is often matched to the size of the chain-rings.  This feature improves shifting. 

if the differences in tooth count are not too drastic, you might be able to get away with only swapping out cassette and/or chain rings.  Also the number of links in your chain may have to be adjusted.