For my exam board WJEC, if I were to get a D in my second year of A level for a paper, then I wanted to re-sit the paper next year and the whole spec changed then they would give us the old spec (so the spec I originally did).
I would be doing the exam with people doing the new spec and people re-sitting who are doing the old spec.
This should be the same for you. So nothing should change with what you learn.
As different exam boards have different rules, I suggest you call or email your exam board. (Alternatively, you could message them on Twitter, usually quicker for me)