Sorry if question has been answered already but I'm trying to change a O/W Award booking using Avios, including the origin and destination cities. When trying to change online, despite suggesting the option to edit O&D cities is there, these appear locked, only allowing date change.

1. Do I need to call in to make this change? If so, any recommended times to call from USA?
2. Is cost $25 per person as well outside 24 hrs?

Anything else I should be aware of?
Many thanks