I am not sure what you mean by an "upgradeable ticket"?
Potentially any ticket holder can be upgraded if it suits the operational requirements of the airline. There is no overriding statute that prevents any class of ticket holder from travelling in a class other than booked and paid for. That doesn't mean that the airline is required to upgrade you to a class of travel for which you haven't paid, irrespective of the circumstances.
Passengers may be marked suitable for upgrade (SFU) if there are likely to be issues on capacity so that the gate agents can narrow down their selection in such circumstances. Likewise the senior cabin crew member can excerise whatever discretion their own company allows if circumstances require an onboard upgrade.
Obviously the airline makes it best revenues form these premium class seats, and people who have paid top dollar for them will not continue to do so if they perceive that they are regularly given to those who haven't paid for them. As it is a matter of perception it clearly helps if the upgradee is a regular customer of the airline, and certainly is dressed and groomed to suggest that they are being appropriately placed. Having said that, there is no guarantee at all of being upgraded unless it clearly suits the airline or somebody with the authority to make that offer.