Yeah, all fish must be taken freediving and must be speared the day of the event. That had been in the rules on our previous site, it seems some information maybe didn't make it during the transition (@Alex Reynaud can you look into that and update the rules post?)

Only one species can be upgraded at each event. So yes, if you have a blue and a ling submitted, and then shoot a bigger blue and ling at the next event, you could only upgrade one that day. Some people strategize different ways... some people who aren't sure they will get to dive at all the events or who don't know they'll fill out the 12 species will enter every fish they get. Other people will only submit fish that are average size or larger for a species so they don't have lots of small fish to upgrade later. Also, note the 5lb maximum for additional weight points, so lots of people won't submit a ling/sheep/etc if it is under 5lbs.

Here is a snapshot of the scorecard in Nov of 2018 to give you a sense of the weights entered for each species: