Wait why would you wait 6 months? I am a bit ignorant on this, but has anyone actually had miles WITHDRAWN from their account like that? As I understand it, if you fulfill the legal obligations in the T&C of the credit card they have no grounds for retroactively removing miles they already posted.

Best move is to spend them ASAP. Then there is nothing to take back.

Edit: It appears 6 months is the rule of thumb on FT, so I did not mean to mislead above.