Christina,

The prefix is designed to be a unique key for that item so in theory we never expected it to change, however you do have the ability to manually edit the key if you enable this ability in the project configuration.

We are going to be building some additional functionality around moving multiple items around. I could see including the ability to modify the prefix to match the group it's being moved into. This would change the id completely in that it would have to also change the number. For example, if you have a System Requirement with a key of REQ-23 and a Feature Requirement with a key of FR-23 when either one is moved into the same group you'd end up with the same key. 

Hope this helps.