This, and technical issues, is why reddit hasn't added this ability in the 7 years it's been around. Even with comments it's happens occasionally, which can be frustrating. I don't see why someone would really need to change the link anyways. If it's a different link it's worth making a new post. That being said, the title is something that I think is worth being able to change.
But you can edit self posts and always have been. CuntSmellersINC used to post heartwarming stories, watch them get upvoted to the front page, and then change them to tales of pedophilia. I myself have said things that are flattering to the hive-mind, then once they've been upvoted to the stratosphere change them to things that are condemning of the hive mind just to get a controversial trophy.