The past - concrete, or mutable? Options

The future is apparently mutable, meaning there are many possible paths that could be taken. Just because we "know" something happened in the past (like a past life, etc), does that necessarily mean it happened, or is just one of many possible pasts? It is common metaphysical doctrine that every event exists in the present moment, so does that necessitate that there is no concrete past?

chaos theory is just a theory.

changing memories is not the same as changing the past, it just appears that way. If i delete a file on my hard drive, does that mean it was never there?

But it can seem like that, it can be useful to alter the memory of time passed by, but that does not change the past itself, actually it only alters current memory, so it changes the present.