Downgrading unlocked Dinc2

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http://forum.xda-developers.com – Probably a silly question but my friend wants me to root his Incredible 2 but its Sim unlocked and he uses it on T-mobile, and I'm don't know if I should root it because I'll have to downgrade it to 2.3.3 and I don't know if downgrading it will re-lock the sim. Sent from my SCH-I535 using xda premium (HowTos)