All travellers need to have a valid travel document (passport or national ID card) with them while travelling.
When activating your Flex Interrail Pass on the DiscoverEU Travel App, you'll be requested to add your ID or passport number. If your passport or ID expires after you have activated your Pass, you'll not be able to change it by yourself. You'll need to contact us to update your document number. If, however, you haven't started travelling yet, you can cancel your scheduled travel days in the ‘My Pass’ section of your app. Then, you can re-activate your pass, through which you'll have the chance to change your ID or passport number.
With the Fix pass, this should not have any impact on your travel ticket. Make sure to travel with your valid document.