Can an unlocked phone be locked after choosing a carrier?

I'm planning to buy an unlocked phone, later choosing a carrier. Then would the phone stay unlocked thus being able to change the carrier freely with a different SIM card? Or is it possible for the unlocked phone to 'become' locked after choosing a carrier?|||No Once the Mobile Unlocked means It cannot be Locked Again....So that Unlocked mobile phone can be Free to use with other GSM network,For Unlocking the mobile Unlock code is best....