Attachment disorders are only diagnosed in children. Once your an adult, if it's severe enough and there was never any help, the attachment disorder could morph into a personality disorder.

There's certainly room for change, it just involves a lot of insight and effort. Attachment styles are ingrained in us from childhood and become part of who we are and how we relate on an unconscious level. So to change them in an absolute sense is probably unlikely and any therapist who claims they can do this is very irresponsible if not an outright liar.

I'm sure think some relational patterns can be changed if that's what someone wants to do and they are really motivated. You just need to be equipped with the tools to do so, whether it be therapy or something else. The younger one is, the easier I imagine it would be since less time has been devoted to interacting and behaving a certain way. I hate to think anything is impossible, but changing an attachment style in its entirety, after a certain point in time, probably is.