Where did I say that those two words have no definitions? Of course the words have meanings, as do all words in any language. The point is that those particular words, "change" and "existence," are relatively broad and general rather specific and concrete. Those are fundamental concepts that are suitable to metaphysics, armchair deductions, religion, and the like.

Your definitions of those two words, though, are quite idiosyncratic. You're not really defining them since the words already have definitions in the dictionary. What you're doing is offering an armchair metaphysical explanation of how these fundamental properties work. (But Kant said "existence" isn't a predicate.)

When you dismiss Platonism for having no "evidence," you're assuming the questions at issue are empirical. But why think the question of why there's something rather than nothing can be scientifically answered? Do you think science can answer all meaningful questions?