iPhone Unlock or Locked After Repairs

I have a iPhone 3GS which has developed the battery issue,
Apple have already said there will replace the handset if bought in,
so have it booked for friday,
what i need to know is about the network lock,
i had the phone unlocked 4 days ago from o2 as i moved to another phone,
i will be giving this one to my Mum,
it being used on T-Mobile at the moment,
So after the handset is replace,
Will the replacement be unlocked or Not??????