The idea of relative usefulness in replacing an old technology is not specifically addressed by the TAM. However, as all of my friends discussed, TAM explains the perceived usefulness of a technology significantly influences its adoption. The new technology should enhance the performance to at least the same level as the old one. People will not embrace new technology if they believe it to be less efficient and functional, and does not provide any additional benefits, especially over an extended period.
According to TAM, perceived usefulness is also influenced by perceived ease of use in the Viswanath Venkatesh et al.: paper. This is because, other things being equal, the easier the system is to use the more useful it can be. In that case, replacing an old technology requires the new one to be at least as functionally useful, while offering a clear advantage of easier use of the technology.