According to this page:
http://www.lifealert.net/services/equipment.html
it can be worn around your neck or wrist. But, it doesn't say that it
comes in a necklace (or a bracelet). It just mentions the necklace.
I'm thinking that the strap is adjustable. But, it would have to be
adjusted pretty small and tight to go from a necklace to a bracelet.