What is the level requirement to change to any of the mentioned class?

You need to be able to find the various items that bestow the talents for each build type. So it varies depending in the build you are aiming for.

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So that means that I will be a warrior forever but will be able to acquire berserker/champion/defender skills from items?

Right! The Warrior has those three builds to choose from. So each of the talents the warrior can get are divided into those categories.
Check out the breakdown here on our SITE:

Thank you very much.

Skills are based on items only. Not stats. An axe will always have cleave and whirlwind.