From a conversation with an English Wikipedia editor user Thryduulf, he says that when it comes to editing existing links (as opposed to adding new one):
there are three things you could want to do:
(1) change the link without changing the text (e.g. [[Mercury]] → [[Mercury (planet)|Mercury]])
(2) change the text without changing the link (e.g. [[Mercury (element)]] → [[Mercury (element)|Mercury]])
(3) change both (e.g. [[Mercury]] → [[Freddie Mercury]])
At present it seems that the visual editor always assumes you want to do 1, and unless you delete the link completely there is no way of doing otherwise..., but for beginners I'd say that 3 should be the default. There needs to be some way to set the target of a link independently of what is displayed, and I'm not sure how best to do that but maybe an option on the link dialog called "display as" or something like that would be the way to go.
Version: unspecified
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=52240
https://bugzilla.wikimedia.org/show_bug.cgi?id=50678
https://bugzilla.wikimedia.org/show_bug.cgi?id=33091
https://bugzilla.wikimedia.org/show_bug.cgi?id=50452
https://bugzilla.wikimedia.org/show_bug.cgi?id=53973