Yes, you absolutely can start your course in the audit track and then upgrade to the verified track later, so long as the upgrade deadline has not passed.
Please note that in order to access graded assignments necessary to pass the course, you'll need to upgrade to the verified track. So, we recommend doing this early on so you'll have more time to work on graded assessments.
You can see the deadline to upgrade on the right side of the Course tab within the course under Upcoming Dates:
If there's no upgrade deadline listed, then the upgrade deadline has passed.
It is not possible to upgrade after the deadline or the course end. If the course's final end date is approaching or has already passed, it will be listed under Upcoming Dates as well.