If you look at this link it seems pretty explanatory. By purchasing this legends upgrade or the the legends upgrade bundle it allows you to pick 1 out of the 5 legends. That would be an additional legend (if you already purchased the HOF edition). That is how I'm interpreting it at least.
https://m.gamestop.com/product/ps4/add-ons/madden-nfl-19-legends-upgrade/163215
If you look at this link it seems pretty explanatory. By purchasing this legends upgrade or the the legends upgrade bundle it allows you to pick 1 out of the 5 legends. That would be an additional legend (if you already purchased the HOF edition). That is how I'm interpreting it at least.