Yes, you do need to update these changes.

As you input your vehicle information, the first page asks about your vehicle.  Describe the old vehicle and when you first used for business. At the bottom of this page, there's a small check-mark box, "I stopped using this vehicle in 2017".  This opens up another question about when you stopped using the old vehicle. 

2.  The next questions are asking about the business use of the old vehicle from 01/01/2017 until the date you stopped using the vehicle.  Answer these questions with this in mind, you will add the other vehicle and expenses once you've completed this area.

3.  Next, TurboTax asks you about trading in your old vehicle and getting the new vehicle.  Depreciation will be figured here.

4.  Then, there's one more step.  Add a vehicle.  This is where your will add your new vehicle and answer the questions relating to it's business miles from the time you began using it in 2017.

Once you've completed these three areas, all your vehicle expenses will be entered as well as depreciation and the information will be carried forward until next year.