Phone Can you change the ROM of every android device?

I am asking this because i got this fake Galaxy S4 gt-i9500 with a Mediatek logo on start up. Its really good so far but i want to change the ROM for reasons you already know ... Anyways i am afraid i am gonna end bricking it because maybe no ROM works for it. Can anyone help me and if it can be done can you show me a detailed tutorial because i am not that good with android os.