An SPG rep finally responded to my questions via Facebook. I asked, "What happens to an old Cat 8 certificate that was attached to a new Cat 6 before 8/18? If for some reason we need to change our travel dates, will we still be able to get the same hotel? Or will we our cert be downgraded to a Cat 5?"
The response was, "If you modify your stay dates after 18 August, you will be subject to the new Free Night Award Chart point values. Extending your stay or shifting your stay dates will result in a recalculation of points required for your reservation."