If you want to get any sign up bonus for a new card you will need to apply for a new card. If you simply convert your old card you will not get any signup bonus which is what I have been told on a few separate occasions. Plus iffor credit score reasons you want to keep the old card but switch to a no fee card then it has to be a United card product. You cannot switch to, for example, a Freedom card. Offhand, I do not know if there is a no fee United card product.