Hi,
No tour operator will sell one-way upgrades until you check-in as they obviously want to sell them return. CSR are check-in will always offer them where available. They will be about £99 OW. They will also be available to book in resort if you require them coming home. Premiair Gold service is superb and well worth the money. On overnight legs the crew tend to not keep asking if you would like drinks as they like to allow you to sleep so just ask if you need anything.
Thomas_Cook_757-300