This is now the latest info on the Portugal Rail website:
Timetable changes from August 5
Since the summer timetable will come into force on August 5th, the sale for the Alfa Pendular, Intercidades, InterRegional and Regional trains will be temporarily unavailable from August 4th. The trains will start operating again as soon as possible.
The sale for the Alfa Pendular and Intercidades trains will be temporarily unavailable from August 5th, since the summer timetable will come into force on August 4th.
I don't really understand what's going on. I cannot buy an online ticket for Porto => Lisboa for any day in the future, either before or after Aug 4. Are the trains not running at all? If so this presents a problem for us as I was relying on a train to get us from Porto to Lisboa (Sintra, actually) on Aug 5th.