You can always substitute, but it's going to alter performance and may require you to change other parts of the circuit as well, not usually adviseable to someone that's learning, unless you don't mind a lot of smoke. A simulator such as LTspice will help you a bit in understanding the circuits you're playing with without haveing to build them and can do some really amazing analytical things as well. You can change the paramaters of a transistor and see the results in a graph on the screen.