there are (at least) 2 things that boycotts are meant to do:
directly deny funding to the company which will pass that onto “causes” you disagree with
make a statement that you and others disagree with decisions the company is making
in the first point, switching to a different brand produced by the same company clearly does nothing unless the at product has a lower margin (which isn’t even unlikely either: plenty of brands do the “budget”, “midrange”, “luxury” brand concept and budget is where they make their least overhead)
on the second however, that’s where you can maybe make a difference… if a company starts to see market share dip in their big name products, that’s problematic for them even if people are switching to other products in their line that are less well known, because it shows that people have more negative attitudes to their brands