How to import userpoints? The User Points module doesn't have migrate integration yet.
Looking at the API I can probably use userpoints_service_add to assign "starting point" values for each user in a batch script.

Is there an option to update the "starting point" value for a user when I go live? Or do I have to calculate the difference and just add an adjustment?
You want to use userpoints_userpointsapi(), not the service wrapper.

The system is designed to only ever allow new transactions in order to change the total amount of points a user has. So yes, if you want to set it to a fixed value, you need to calculate the difference from the actual amount of points a user has and grant him the difference.