Nope.

Please elaborate; I would like to know why you disagree. Please try to prove your arguments.

Ugh, look I'm assuming this is a word salad because it reads as if it's trying too hard to be profound, but if I must:

"Before becoming" it isn't a thing at all, so it is neither what what it will become nor what it will be, nor is it even itself because it isn't a thing, it has no properties because there is no "it" there.

The "thing" is memory, and memory is persistent. It morphs but also retains.

OK. So you have recapped the gist of Ancient Greek metaphysical dilemmas. Let's jump to the resolution.

Individuation is always true from some point of view - some scale of observation. The world is a process. It is always either relatively individuated in some fashion, or relatively not individuated. But for the sake of simple world modelling, we like to construct a sharp dichotomy that separates all things into two categorically opposed baskets - flux and stasis, change and stability, potential and actual.

We can begin to see the fluidity of this interplay - this oscillation between the limits of two poles of being - once we understand every individual, every substantial being, in a multiscale fashion. That is, a hierarchically-ordered point of view which spans all the "cogent moments", or integration scales, of any entity that appears to exist in space and time.

So look over there. I see a river, a mountain, a wave, a wind ripple passing through the grass.

The mountain must have existed forever - from our typical human-scale perspective that also sees the wave as not really a proper entity at all but just a momentary disturbance in the water.

But if we had eyes to watch a landscape over millions of years, we would see a geography as fluid and turbulent as the surface of the sea. The rivers would snake and twitch, disappear and reform, with even greater abandon.

Likewise, if we zoomed in on any fleeting entity, its existence would start to stretch out so that it seemed completely permanent and substantial compared to our scale of individuation.

So a modern metaphysics would see everything as a process. And what differs is the characteristic rate at which some part of the world becomes individuated from the general backdrop in some significant fashion - before disappearing back into that backdrop.

Any thing can exist only once — Daniel

This depends on how you define the concept that is represented by the word "thing". If it's too specific, the object will exist for a very short period of time (i.e. once.) If it's too generic, the object will exist for a very long period of time (e.g. forever.) Depending on how you define your words, you can say that everything exists only once or that everything exists for all times.

which happens when it becomes; after becoming, it cannot be anymore, for it is subject to change, and necessarily "becomes" something else. Before becoming, it certainly is not the thing that it will be. So, it is as if something exists (potentially) up to the point it becomes an actuality; after that, it "becomes" literally nothing, for the memory of it is totally different to the thing itself. — Daniel

Your concept of "thing" goes through three stages: potentiality, actuality and death. Again, this is a matter of definition. It's not an empirical matter.

Everything is in flux, we just choose to call certain, I suppose, "significant coagulations that are seemingly persistent in time" ---> "things" (However, do ideas count as "things" in your scope of the word?)

This depends on your definition of a "thing". If you want, you can define a thing to be a moment of time. One second? Nano second? What scale? A definition is only useful, as long as it can be accurately, and conveniently, applied to reality. Such a definition of existence fails as a tool, and is useless in application.

Yes, ideas do count as things in my definition of "thing"; in fact, everything that possesses a limit I consider to be a thing. And I actually consider ideas to be an excellent example of such phenomenon -that everything can exist only once, for I believe that and idea cannot be the same twice. An idea can be similar to another, but never the same since the idea thought first by being first in thought is already different to any subsequent ideas. And this I believe occurs with reality as well. Not a single component of reality can be the same twice except if it does not undergo change. But I am not able to think of anything that does not undergo change. Even change, which remains constant, must change in respect of itself, at least, since it is not possible that the change that led to this moment be the same to the one that's led to this one. They differ by succession. But, if everything is different at every single moment, what makes identity possible?

This is why I've never liked mereology and would rather look at it from a linguistic perspective. Staring too hard at the metaphysics of identity leads one to counter-intuitive or unhelpful conclusions. Fundamentally, I believe I agree with you to a degree, but the language of identity is needed for us to operate in the world. In this case you could say there is only one "thing" the universe, though as it changes its energy is conserved. However, what do you think of natural numbers and their identity? Does the number one remain the same over time, or do we all have a slightly different concept (maybe not a concept, maybe a phenomenological impression) of the number one, two, three, etc.