When you click "Add Milestone Start Date" you create a new registration period segment, pass prices will automatically change at 00:00:00 (midnight) on the first day of the next period. If you want an early bird price to end on 31 December, create a new Milestone on 01 January.

Once a registration segment has expired, it cannot be extended. You will need to add a new milestone or milestones and update all pass and group pricing accordingly.

You can set up pricing changes on the Tickets or Pricing Groups & Codes screens.

Under each group (on the Tickets screen) or under each ticket (on the Pricing Groups & Codes) screen there is a section for each registration period with a separate price field. In the price field, add the correct price for every segment of the registration period for each type of ticket.