Personally speaking, the fact that you can acknowledge to yourself that you're not ready right now is awesome. In my own life there have been times (non CF-related) where my ego has taken precedence over my common sense and I've ended up blowing or significantly delaying the goals I had set out to achieve.
If at all possible, don't wait until you have confirmation to refresh. Do a little as often as you can, have a schedule you know you can stick to (i.e. 1-2hrs 3-4 times a week or whatever best suits your life-style at this time). In my opinion, it's better to have a solid grasp on the subject that has been slowly retained and can be applied calmly and confidently when your nerves might have the best of you (i.e. the test), rather than to cram as hard as you can in whatever time-frame they've given you. (There's a possibility you could find out you're eligible, then learn that your test date is only a couple of weeks away) I'm sure you know that work, family, etc. can take over at times and before you know it, that time frame is over.
As well, if unfortunate news arises that you can't re-write, at least you know you've significantly improved in an area that will be beneficial to future endeavours. Best of luck!