Certainly there is no particular evidence for a change of name. Taylor's* map of Montgomeryshire (1718 CE) shows the Severn back to its source. Coles* notes that the Trannon was known as the Tarannon in the late 16th C, though doesn't discuss its relationship with the Severn. If Cole's thesis about the name being related to the Trisanton[a] names is correct (See Note) it is possible the name could have applied to the Severn more generally, however there is apparently no other reference to the Severn using this name. The name Sabrina, which is almost certainly the Severn, extends back to Ptolemy (2nd C: see Other Works), while Geoffrey of Monmouth (Historia Regum Britanniae ii.5: see Other Works) quotes the legend of Habren, who he suggests gave her name to the river, and further suggests the name was corrupted to Sabrina (The Welsh name is the Hafren: Geoffrey seems correct in his suggested corruption - for details of the conversion to "Sabrina" and the English "Severn" see here). Incidentally, the legend is that Locrinus, one of the three sons of the first King of Britain, Brutus, became ruler over Loegria, that is, approximately England. He fell in love with the King of Germany's daughter Estrildis, with whom he had a beautiful daugther, Habren, however, he married another woman Gwendolen for political reasons. When he finally left his wife for the German, Gwendolen raised an army and killed him. She then drowned Estrildis and Habren in the Severn, but stipulated that the river should take Habren's name in honour of her.