You don’t have to have new ones, especially if the bands are less than 2-3 years old. The bands have 2 rfid chips in them, one passive that will last basically forever and is used for park entrance, opening resort room doors, paying for purchases etc., really anything where you have to touch the band to a reader.
The other is a battery powered rfid that is used for longer distance things like identifying you for pictures on rides. It’s battery last about 2 years according to Disney. So, if you want those things and the band is over 2 years old then you might consider a new one otherwise not necessary.