Can I change my trade ports?

With the Man the Guns DLC it is now (2019) possible to control the routes used for trade convoys. You can indicate that a sea zone is avoided if possible or even forbidden in all cases.

I have used this feature to route convoys out of air range and away from contested sea zones. That has indirectly changed the destination port for convoys. But I can't confirm that it will change the departure port.

It doesn't look like you are able to change trade ports manually. See this post:

Trade routes are automated.

I have found nothing to indicate that you are able to change trade ports, so it looks like you're stuck with whatever it chooses for you initially.