Reading the various threads and FAQs on the SPG forum, this is what I think will happen:

Most likely scenario: The certs will map to a new category/point level for the same # of nights. So for example a current Cat 1-5 might map to 7 nights at a (new) Cat 1-4 hotel OR be points based (i.e. 7 nights at any hotel up to 25,000 a night) similar to the credit card certs that they will be offering (e.g. 1 night at any hotel up to 35,000 points).

A slightly less scenario might be a cancellation of the certification and redeposit of the face value points into your account i.e. 150,000 points for the current Cat 1-5 cert. This will obviously be huge for us but the only reason I think this may be plausible is due to IT limitations and not being able to roll out anything new before Aug 1 (with everything else they have to do). So the easiest way would be to manually cancel the certs and re-deposit for points (will be easier to get people to manually do this then to roll out an IT solution by Aug 1). But as time goes on and people figure out the arbitrage opportunity, this scenario is becoming less likely and/or the there will be no notice for us to definitely know and take advantage of it.

The least likely scenario would be that the current Cat 1-5 cert becomes a 150,000 point cert for use with any reservation for any # of nights as long as the total stay is less than 150,000 points. Don't see them doing that and this would require the same (or more) IT work than the first likely scenario.