Can I use old Apple Watch bands on the new series?

Apple Watch bands are interchangeable between different series, however you must check the case size. Apple Watch bands can be swapped as follows:

• 38mm bands fit on 40mm models
• 42mm bands fit on 44mm models

Starting with the Apple Watch Series 4, case sizes increased 2mm. All older Apple Watch bands from the initial release until current styles are compatible with the latest Apple Watch. Just make sure to check the case size above.