How to update a iPhone 4 from 4.X.X. to 6.X.X. jailbroken without losing baseband?

Hi peopple,

I'm with a situation and I would really appreciate some lights. So thanks in advance.

I have a friend that bought her iPhone 4 when it lauched but she changed the carrier. So she went to a guy that "unlocked" the device. The thing is she has her iPhone with 4.X.X. and she would like to update to a more recent version (6.X.X.) without losing the baseband. And beeing able to unlocked the device for other carriers.

her current baseband seems to be: 01.59.00

Is there anyway to do it? Could it be possible?

Thanks a lot guys!