Shapeshifting in Old Norse Religion as well as other native religions throughout Northern Europe and into Siberia is known but perhaps not spoken of enough about its importance with the practices of Shamans, Witches and with the use of Seiðr. We do see shapeshifting in the stories of the Norse Gods and Goddesses mainly with Odin, Loki and Freyja but not as much with humans like Fafnir the Dragon from the Saga of the Volsungs who used to be a man. Like what we can see with other ancient cultures and even to present times, Shamans will dress in animal hides and become Therianthropic during rituals which is a symbolism of shapeshifting itself. An example that is quite well known during the Viking Age are the fierce wild warriors known as the Berserkrs and Úlfhéðnar which were special warriors that dressed in the hides of Bears or Wolves and they themselves became animalistic.
Besides what was written down or passed by word during and after the Viking Age, as well as other Northern regions across Europe and Asia, regarding shapeshifting, there also is some physical evidence of its spiritual importance with such examples as the Khakassia Petroglyph in Siberia. Shapeshifting depicted with Petroglyphs can be found in other parts of Europe, Asia and especially the Amerindians of North America.
Personally I find the subject of shapeshifting quite fascinating and have read much about it as well as watched a number of documentaries that touch on the subject. Below you will find a selection of resources I highly recommend having a look at.
Three Awesome Magical Powers in Norse Mythology