I think we should embrace what many philosophers take to be a plausible view regarding truth if we are committed to propositions as truth bearers, namely, its omnitemporality: If a proposition is true it is omnitemporally true: it always was, is, and will be true. What changes is our estimation of the truth not the truth itself.
Some might say that propositions about the future have no truth values yet. But if propositions about the future have no truth values then arguments that make future predictions would be impossible since all the propositions in an argument are truth claims. Richard Taylor provides a helpful elaboration in his book Metaphysics (Prentice Hall, 1992): “A prediction, to be sure, must await fulfillment, but it does not thereupon for the first time acquire its truth. Indeed, had it not been true from the start, it could not have been fulfilled, nor its author congratulated for having it right. Fulfillment is nothing but the occurrence of what is correctly predicted” (66). Moreover, we think it was true before humans existed that pure water freezes at 32 degrees F. Did the proposition ‘pure water freezes at 32 degrees F’ become true only when it was discovered or stated to be true? Surely not. But perhaps most importantly, if we think truth is something that can change with changing circumstances then we run the risk of endorsing a form of relativism where “truth”, rather than being something objective, is nothing but the opinion of a certain individual or group of people. Once relativism is accepted then there is no objective foundation for critique of any sort.
For more on the nature of truth, go here.