Out of curiosity how does one upfare a ticket instead of purchasing an upgraded seat? Can that be done through the website or do you need to call in for that?
If it's a corporate ticket you can just call in (and they'll usually waive the fee to takeover the ticket from AMEX GBT or whatever TA your company uses)
If it's a personal ticket you can do it online now because there are no change fees.
An upfare is just paying the difference between the cost of the Y ticket and the cost of the F ticket so you of course can just do it at booking too (WFBF). It's more useful for corporate tickets where company policy might dictate Y only but you end up with fairly expensive Y fare classes to the upfare to F is a minimal out of pocket expense for you as the actual traveler