I see the page for SWT Weekend First has changed to be "fares from £5", but without any details of what fares will be charged if someone wishes to upgrade at weekends.
I reckon it might backfire in a way, especially for those travelling in a group who would pay extra for weekend 1st class on say a service to Bournemouth
I've checked a few destinations in Avantix Traveller and it appears to be £15 to Exeter/Weymouth/Baskinstoke/Southampton/Pourtsmouth, £10 to Salisbury/Bournemouth and £5 to Woking