Im going to buy a 12 wk old puppy but its already named,if i want to change the name will it forget the other ?

Yes it will forget the old name. I adopt a dog at 1yr that was previosly named Lia...I personally didn't like the name so I decided to change her name to Lola...after about 2 weeks she began responding to her name. Dogs are very smart animals and adapt very fast. Just always call it the new name and it will quickly learn the new name