For one of the games I speedrun, there's no subcategories. But right now, we're planning on making some. The problem is we don't know how to transfer the already existing runs to those subcategories.

Go to the variable that has those sub-categories. Then, click on the 3 dots, then click "Set the default for all runs that don't have a value". The runs in said category should appear.

Not all runs will fit in the default value all the time, so you'll have to go in and set those runs to the right sub-categories.