This question was asked in 2010 here: How to change the link of a BITLY link
Is it still the case that there is no way to create a bitly link first and only later set the longURL for it? This helps in applications where you want to show what the bitly link will look like before the exact longURL is known and set.
Still not possible: the documentation in the Bit.ly API for user_link_edit has no option to change the linked URL.
You could use another link shortening service that allows this, for example the tiny.cc API (tiny.cc/api-docs) has an edit method which allows the URL to be changed.