Global: (although they also have a problem differentiating between a "premium fare" and a "total rip off").
snicker 
From spending time as pax in both Biz and Eco, as both corporate paid and self paid (again, in both) I see no evidence that upgrading encourages people to buy the premium cabin next time.
Friends of mine who have travelled a fair amount and have the money to pay for (say) World Traveller Plus - but they only pay for Y. On one occasion they were upgraded long haul to WTP. Were they impressed? Not a bit. In fact it reinforced for them that they would not pay for WTP!!
Parents of another friend were going to Israel (I think) on a charter and paid for the premium cabin. Which is mainly space, not service, I gather. They said, "Well it was quite roomy but we didn't think it was worth it and won't do it again."
Conversely, I pay for Premium Economy on my own account and do consider it worth the money - who is to say?
If I was a carrier - I would give very few upgrades. If folks always pay Y, then that is what they can afford or want.
The people to try and locate, I should think, are those who regularly travel with you but have to be in Y, as their companies will not pay for them to sit further forward. These are folk who have to be on your plane and can become big fans when treated right. Locating them could be difficult!