Even though Sweden is basically a cash-less society, there are still quite a few people that use cash in their daily lives.
However, the Swedish National Bank (Riksbanken) has recently changed the bank notes for 20, 50, and 1000 krona with a new design.
You have until the 30th June 2016 to spend your old notes or deposit them in your bank. After that date, they will invalid and rejected by stores and sellers.
You can learn more on the Riksbanken website.