"In case you didn't know" is talking about what you knew in the past. "In case you don't know" is talking about what you know now. That being said, it is true what Mr. Zhang says. I am not aware of any Native English speakers in the US who don't use these interchangeably. I have always used them interchangeably and was never corrected by any of my English teachers... so I assumed all was well!! :)

No. One means past, "In case you didn't know, l was late to class" The other is present state "In case you don't know, l am here in class".

Hi Sinem, You can use both interchangeably in conversational English now. This is a case where the subjunctive mood (used to express possibility or doubt) in English is disappearing, so "don't" and "didn't" both work here.

Yes, they can. Because of their tenses, you'll more often hear, "[explanation], in case you didn't know." and "In case you don't know, [explanation]." Rather than the other way around.