I didn't ask you to define "existence." Why would I need you to do that? I have a dictionary. Scientists don't just define words. They explain what things are by testing hypotheses.

You can redefine words however you like, but you should be aware that the results will be semantic. What can be trivially defined can be trivially dismissed since someone else is free to redefine that word. That makes for only a semantic disagreement. Both philosophy and science are more substantive than that.

I didn't say your definition of "existence" is abstract. It's the common English one that's abstract. Your idiosyncratic one is just arbitrary. If you want to know what my definition of "existence" or "change," you can consult the dictionary.

My philosophy isn't about defining words and playing word games. I try to understand things and to formulate a big picture by pulling many things together, using arguments and explanations.

"Scientists have established that a changing natural universe exists." is not an abstract statement. It's a plain, general fact. Scientists posit a natural order, and they discover how nature works. This is a general fact but that doesn't make it abstract, not according to English.

I don't say that science alone explains everything. I'd add philosophy. And I'd add religion if we're talking about the collective fictions or myths that comfort us in light of what reason discovers.