https://www.nationalrail.co.uk/times_fares/ticket_types/46557.aspx#Weekend_FirstOn weekends and Bank Holidays, upgrading to First Class on Virgin Trains is free of charge if a Standard Class Anytime ticket is held.
"Buy a Season Ticket for one month or longer and you can enjoy free First Class upgrades on your normal daily TransPennine Express route at the weekend (excludes Bank Holidays)."I think season ticket holders can.
Having said that, I once had one of their train managers decide use a British Transport Police officer to remove me from the train (which, obviously, was a success) for taking them up on this offer, which the train manager claimed never existed.