| I am just trying to say that it 'could' be said that a known truth at any point in time can become something else later on. |
There are some limits on the degree to which 'truth' is becoming 'something else'.
To use the flat earth argument, in order for a flat earth theory to supplant the current state of knowledge, it would have to subsume *ALL* of the physical characteristics and observable phenomena that are currently included in the current viewpoint. Additionally, in order for it to become accepted knowledge, it would also have to predict events or behaviour accurately that cannot be predicted or explained in the current system.
To use a simple example, take an eclipse (solar or lunar). notice the shape of the shadow on the moon for a lunar eclipse, or look at the shape of the occluding form blocking the sun in a solar eclipse. In every observed instance, that shape has always been circular, regardless of when they are observed, and the relative positions of the subject bodies. As a brief foray into elementary geometry will make clear, the only geometric form for which every cross section (or projection, for a shadow) is circular is a sphere.
Apart from this, there is a vast amount of observational evidence (and theoretical as well) to support the sphericity of all sufficiently massive bodies.
A flat earth theory, to be valid, would have to subsume (incorporate) *every* bit of the above observations and theoretical framework, and it would have to improve on the current theory as well (with predictions, bourne out by observation, that cannot be made with the existing theory). If it is only as good, there is no compelling reason to choose one model over the other.
I said all of that to say this: A large number of people (some of whom have been taught in science) fail to understand the mechanism of the formation, development, and growth of scientific theories. Where a scientific theory has been bourne out repeatedly by a substantial number of observations and validated predictions, it does not just go away once a case has been found that the current theory cannot explain. What happens is that the new theory that explains the new problem still has to explain (fully) all the observations and validated predictions that the old theory explained. The old theory doesn't go away, it essentially becomes part of the new theory.
To elaborate with an actual occurence of such subsumation, I offer Newton's laws of motion, and Einstein's special theory of relativity. Simply put, Newton's theories stated that the velocities of motions were linearly additive (like throwing a 100mph fastball forward from a train moving at 50mph, the ball would have a groundspeed of 150mph). Einstein's theory was written to explain characteristics of motion (theoretical at the time, but subsequently verified experimentally) where the linear addition of velocity was no longer valid (very high velocities approaching the speed of light (to abuse the above example, we'll launch a baseball at 50% of the speed of light, from a spaceship that is travelling at 90% of the speed of light. The baseball will have a measured velocity that is *less* than the speed of light, somewhere around 99.999%)). Einstein's laws of motion produce the same results as Newton's for relatively small velocities, it only differs in the exceptional cases. It also makes verifiable (and verified) predictions that Newton's theory cannot make.
Such incorporation is typical of well established scientific theories.
Now, when you start discussing very novel scientific theories, philosophical 'truths', and historical assertions, it does become possible for a subsequent theory to have a substantial chance of fully supplanting a prior viewpoint. Even then, the supplanting viewpoint needs to make a better fit of the data and evidence than the previous viewpoint (or as is often the case, incorporate new evidence or data that is at odds with the prior view).
