Beazelbub
I am not sure what you mean by an "upgradeable ticket"?
The OP is talking about fare class, as many airlines (e.g Lufthansa) restrict voluntary upgrades to certain higher tariff classes, e.g. Y and C.
In the US, certain classes are often given upgrades to First Class based on FF status and fare class, subject to airline discretion.
Other airlines, such as OK, openly offer guaranteed upgrades based on FF status, fare class and availability.