For those of you asking about net worth, I believe, for the purposes of the challenge, we're using the property value that is displayed within the Mansion Baron path of the Fortune Aspiration. The rules have been modified to allow you to switch to Fortune>Mansion Baron periodically so that it updates for you. So if you're currently working on, say, Creative>Painting, you can switch out to Fortune>Mansion Baron and then switch back to Painting.
Keep in mind, many of the the values don't update until/unless you make Fortune>Mansion Baron your active Aspiration. That's why the challenge now allows you to switch to Fortune>Mansion Baron without penalty.
Other than that, you may only switch Aspirations when one is 100% complete OR when one is impossible to pass because it contains requirements that are not allowed in the challenge (for example, it requires you to get to level x in a career).
@moongodess I hear you about watching a Sim paint. It's like the old joke about watching paint dry, but we're actually doing it! At the very least, I wish Yuni (my challenge Sim) would turn out more masterpieces. These 100 simolean paintings are driving me nuts. It's like she doesn't realize she's in a challenge!