The WFRP2 core rulebook is a little ambiguous on this point, depending on whether you want to go by a literal reading or to read the text in a more natural manner.
According to "Changing Careers" on p.28,
Every career description has a list of Career Exits... The first step towards changing career is to pick which one of these exit options you’d like to take.
Before you can enter the new career you’ve chosen there are two
things you must do. First, you must collect all the trappings of the
wish to join. Second, you must pay 100 xp to enter the chosen career.
There are "two things you must do" to change career, neither of which is "buy all the advances available from your current career." You can take a career exit at any time, provided you have the requisite trappings and 100XP to spend.
Additionally, the "Career Exit Options" sidebar on p.29 states that, when changing careers, you can use the career exits from any of your previous careers, without specifying that you must have completed the career whose exit you're using.
Note that, while WFRP2 can be read ambiguously, WFRP1 explicitly stated:
Characters may normally change careers at any time, provided they have the 100EP to do so. In some cases, however, it is necessary for characters to have taken every advance and skill available from one career in order to progress to another. For example, wizards' apprentices must complete their advance scheme before becoming a wizard.
In addition to the sentence cited in the question, p.30 says
When you finish this career, you can take any of these exits for 100 xp.
In common usage, this phrasing implies that you cannot take those exits prior to finishing the career, just as "when you finish dinner, you can have ice cream" implicitly makes finishing dinner a prerequisite for having ice cream.