The 2020 FoE Soccer Cup Event added the possibility to upgrade this building.
The Winners' Plaza can be upgraded by Winners' Plaza Upgrade Kit item. To upgrade the Winners' Plaza, use the Winners' Plaza Upgrade item on the Winners' Plaza that is already situated in your city. Its upgraded version will then be placed directly on the same spot in your city and the production timer will not be reset. Note that the upgraded building will be from your current era, even if the old one was from one of the earlier ages.