Is there a mechanism for transferring attachments across a single Player's characters, or even between different Player's characters? I have been playing as if a character is stuck with an attachment until he/she leaves play or a specific card effect changes things, but I want to make sure I am not overlooking some additional flexibility. I didn't see it mentioned in the Rulebook or FAQ, so if it's there I apologize.

Attachments cannot be transfered unless they say so. If an attachment is removed from a character by any card effects it is discarded,

so before you attach an attachment to a character be sure you won't change your mind!

Thank you, Ruchlas!

May I voluntarily discard an attachment??

No. The only time this is possible is if you're playing a 3rd Restricted attachment on a character - at that point you can choose which of the first two Restricted attachments you'd like to discard to make room for it.