The bands have an RFID chip in them. That chip is linked to your account on DME. Once you change something on DME, the RFID chip is changed.
My husband and I have MagicBands and annual passes. So we get to keep our MagicBands all year long even though we make four or five trips a year. For our Thanksgiving trip, I was making changes to our FastPasses just days before we left and we had our Bands since April. All was good.