Question about changing Android versions

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http://androidforums.com – Using Samsung Galaxy Note SGH-T879 (the Tmobile edition of the i717) with Android version 4.0.4. I will frame this questions in terms of something I understand fairly well, Windows OS on a PC. Is the ROM on a smartphone the counterpart to Windows on a PC? Is changing Android 4.0.4 on a smartphone to version 2.3.5 the same as changing a ROM? (General)