Absolutely. Depending on what pre-built you have, you could have no good options for that type of upgrade. You are really only saving the cost of a case and power supply which are likely very cheap units to begin with. Components in a pre-built system typically aren't high quality... It would be best to just bite the bullet and buy a new case and quality power supply.
If you have a budget in mind and can list your current components (memory, HD , SSD, GPU), the forum would be happy to get you some proposed builds.