Surely I'm doing something wrong, but I can't find how to refit some fighters to an improved version of them (the tonnage difference is less than 1%).

At the planet where the fighters are in orbit, I select one of the Repair shipyards, then select the task Refit or Auto Refit, but the lists in Refit From, Class and Ship Name are empty.
The other tasks work well.
What am I missing?