I've changed my mind.
'Cambiato' — cam-BIA-to. Stress on the second syllable. 'Idea' — i-DE-a, three syllables, stress on the second.
Use when you have genuinely reversed your position on something. In Italian conversation, admitting you've changed your mind is respected rather than seen as weakness.
'Cambiare idea' (to change idea = to change one's mind) is the standard Italian phrase for changing one's view. Note: Italian says 'idea' (singular) where English says 'mind'. The passato prossimo 'ho cambiato' marks a completed change.
Mi sono ricreduto/a.
I've revised my view / I was wrong.
'Ricredersi' — to revise one's belief. Implies you were wrong and now you know it.
Non la penso più come prima.
I no longer think the same as before.
Process of change — implies a gradual shift rather than sudden reversal
Ho riveduto la mia posizione.
I've revised my position.
More formal — 'rivedere la posizione' is standard professional language for position change
Italian culture distinguishes between 'voltagabbana' (turning your coat = opportunistic flip-flopper) and genuine intellectual change of heart. The former is scorned; the latter is admired. The key difference is whether the change is motivated by reasoning or by self-interest.