The only type of discounted First Class upgrade that can be purchased on some trains is the 'Weekend First' offer which some (but not) all train operators provide. It's only available on Saturdays, Sundays and Bank Holidays.
Any journey from North Wales to Glasgow is likely to commence with the services of Arriva Train Wales, who don't offer Weekend First.
The change at Crewe would take you onto a Virgin Trains service, where the policy is as follows:
"Upgrades are priced between £10 and £25 on Virgin Trains routes, you should talk to the Train Manager for prices for your journey"
http://www.nationalrail.co.uk/times_fares/ticket_types/46557.aspx