You were told that todays theories are unlikely to remain in the their current form 500 years from now. "table salt" "Chlorine" and "Sodium" are definitions, not theories. The fact that Chlorine and Sodium combine is also not a theory it's an observation.
You are talking as if the major theories of science today are in fact entirely wrong, and will have to be replaced in the future by completely new and different explanations. Larry has been talking like that as well.
If that happens it will be the first time it's happened in the era of modern science. It's never happened before. What happens is that the theories get improved and refined as we are able do more research with newer & more precise instruments ... but so far the basic theories have remained the same. And we have been through all this before in this very thread.
If you wanted to say that all of our theories are approximations to some degree, that might be closer to being correct, and actually upon the basis that absolute certain appears to be impossible in a universe that is based at it's heart upon probabilistic quantum events (which what we currently think is probably true, because all known evidence agrees with that) ... so in that sense any explanation or theory will never be a 100% certainty ... they will always be approximations of some sort, albeit approximations to an accuracy of 1 part in a million for many of the processes that they describe.
So if you were being honest about it, then it would probably be more truthful to say that our current theories are extremely accurate and will probably get even more accurate in the future.
The theories that explain how why Chlorine and Sodium exist and how they get built up from ever smaller subatomic particles are can and probably will change. These new theories will have to explain current observations like Sodium and Chlorine forming Sodium Chloride.
That would not change it from being NaCl though.
The macroscopic solid that we call Sodium Chloride, would, even according to what you just said, still be composed of one part of Sodium bonded to one part of Chlorine. Or more accurately it would consist of one part of Sodium 1+ ions ionically bonded to one part of Chloride 1- ions.
What you seem to be talking about is subatomic particle theory, where you are saying that our description of subatomic particles may change in the future. Well, it probably will change, though it's hard imagine how it will change other than becoming more refined and more accurate … but you would still be left with a description where the subatomic particles composed individual atoms, eg sodium atoms which could then exchange electrons with other atoms such as chlorine atoms (so that you get Na+ and Cl- ions sticking together).
Most scientists would not rule out anything. And I'm not ruling out anything. But that's very different from you trying to imply that our current understanding of chemical compounds such as NaCl is entirely wrong or entirely just a useful but false representation.