My sims had one BFF is Bella Goth, my sims haven't call her for a while. Suddenly, one days Amber is particularly good with my Sims and pop up the BEST Friend option. So, i click that and my challenge have 2 BFF now. But when i check the friends status, which you can click the panel and check the picture. Bella Goth is no longer my sims best friends. So i figure it out , the way is you have another BFF, then you don't contact them anymore and let the relationship bar drop. and just keep befriend with others... one day your BFF is no longer fill that post and you can click to invite others to be BFF. But still they are your good friend or friend....... The relationship go low fast in a few days in Sims4.... so it take some times that your BFF change.
Anyway this works for me , now my sims have 4 BFF now, now i just need to wait and will have 5 later on. i am hoping that i can finish tonight. This one is so hard.