I want to re-book guests who are currently staying but the calendar only shows this year and next.
Go to Options > Settings > Bookings tab and in the Max Years field change the 2 to 3 and Save Changes.
This will allow 2019 to display.
Make sure you have entered the tariff season dates for at least January and February via Calendar > Seasons otherwise the base price will be used when a booking is made.
If you do not want consumers to be able to book this far in advance on your website make sure you use the Block Beyond field under Options > Settings > Bookings tab