This is my rational for doing it now:
I am a SPG member so can redeem 90,000 SPG points for 270,000 Marriot rewards points. I can then get 132,000 UA miles + 7 nights at any Cat 1-5. After Aug 1, I believe that all floater certificates would be converted back to points or at the very least will be converted to a point based certificate. Either way we are talking about 150,000 Marriott points "back" into my account in one form or the other. So basically I redeemed 120,000 Marriott points (or 40,000 SPG points) for 132,000 UA miles. Cant beat that and I doubt that redemption ratio will remain, but just my 2 cents (maybe it will remain!)