My understanding (and experience) is that you need to book a new trip with the value of the trip credit or higher before it expires and that the new flight must be after the original trip credit's expiration date. You then need to wait until the trip credit would have expired before canceling the new flight. If you cancel the trip after the original trip credit expires, you will be issued a new trip credit with an expiration date one year from the date you canceled it—effectively extending the credit.

If you buy a ticket for less than the value of the trip credit, you will be issued a new trip credit for the difference between the flight you purchased and the trip credit you used to purchase it with the original expiration date. Everything I said above should apply to the new flight you booked. My understanding from reading this thread is that if you cancel the new flight before the original trip credit would have expired, you will be issued a trip credit with the original expiration date.

Folks should feel free to correct me if I'm wrong about any of that. I should also say that it's also my understanding that there's some mixed experiences and missing data points related to how the old/new tickets were purchased (e.g., entirely with trip credits or with a combination of trip credits and CC to pay for the extra) and some heterogeneity in folks experiences. So there's always some risk here and reports on what works and doesn't would be very welcome.

Best and safest is definitely to book a flight you plan to take before your TC expires. If things don't work out and you have to cancel or change though, please do report back! Good luck!