"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 any hard evidence. .
Is a fact the truth? It depends on what the fact is pertaining to, for example: Many Christians believe that if you do not surrender to Jesus/God you will not go to heaven. Is that a fact; Yes. It is a Common Truth; therefore it is a Collective Fact. This circumstance also covers another type of truth, Faith. Is there a God or divine being? This can be argued to be categorized as Faith: Believing/hoping something is true, therefore in your mind it is fact. Second category, Collective Fact: You have been told this is true all of your life by all the people you love and respect (a collective) so it is embedded into your way of life that it is true.
Scientific Truth: Undeniable evidence to uphold the truth. This truth has been proven time and time again trough scientific testing therefore making it the truth, a fact. This is usually intertwined with a Research Fact, because it has been studied by an experienced person in that field validating the fact. The great disadvantage to the scientific truth is that it is limited by its" resources. It was once thought that man could never travel greater that then the speed of sound, at that time that was a fact but that was because we did not have the knowledge or innovation we currently have. We first had to develop the jet engine and then constantly improve upon it. .