“Can a "fact" change or are facts relative to the knowledge we have at a given time?”
Facts are indeed relative to the knowledge we are given, for example: Earth is the only planet that sustains living organisms. For this period in time Earth is the only planet with life, so we say; because that’s all we know. It has not been proven, without a doubt, that other planets have inhabitants, fact. We would be naive to believe that Earth is the only planet with life, but nevertheless we haven’t