Mobile exchange in another branch possible?

Yesterday I signed a new contract and I wanted to take the I phone 6s or 7. The seller, however, said that this was not possible and recommended another (Huawei p20 lite). Now to my question, is it possible to exchange a mobile phone which has not yet been opened at another branch like Media Markt or Most Wanted in order to receive the desired mobile phone?

Why should they do that? If you had bought it there, that would be something else, but they do not make any sales with you by the simple exchange, on the contrary: they pay even more on the worse exchange value…