Does this sentence make sense?
It's not so late that you can change your mind.
If yes, how different is it from the following sentence and how can we explain the difference?
It's not so late that you cannot change your mind.
If you wanted to use a positive verb you could say 'It's not too late to change your mind'. this too is a natural sentence. Perhaps this is the source of the apparent paradox that seems to have been bothering you.
1. "It's not so late that you can change your mind."
Why doesn't this make sense? Well, you could force it to, but it's complicated, and not very convincing.
By analogy with:
2. "It's not so late that you can't change your mind." meaning
3. "It's not too late for you to change your mind", (as Bob pointed out) then 1. means:
4. "It's not too late for you not to change your mind."
Now, what does 4. mean?
Consider the opposite of 4:
5. "It is too late for you not to change your mind." ie.:
6. "It is time that you must change your mind." Now, the opposite of this is:
7. "It is not yet time that you must change your mind." =
8. "You don't have to change your mind yet."
So, 1 = 4 = 7 = 8
However 1. also means 9. "You can't change your mind yet", so it can't mean 8.
If you can find the logical flaw, or explicate a definite meaning for 1, then please share.
If not, let's assume that 1. "It's not so late that you can change your mind." is not worth pursuing as a useful sentence.
your explanation is as enlightening as always. You are a great teacher, and if you don't mind, let me tell you that your avatar suits you.
As for the original question,
Thank you very much for your enlightening reply. Now it's crystal clear. :)