That would be awesome if it worked. Are you sure that it is MR > SPG at 1:1 though? I thought it was going to switch to MR > M(arriott)R 1:1 - I think it's possible that the MR > SPG disappears on 8/1. But now I'll be on the computer on 8/1 seeing if that MR > SPG transfer at 1:1 exists!
No, I'm not sure that it'll work.

That's why I've said it's a possible loophole

And tried to limit my thoughts to non-definitive phrases
But here's my take. The Summary of Changes, effective August 1, 2018 states (and if there are typos, they're mine. i'm typing it out from my paper notice cuz i'm not smart enough to find where the change notices are in my account):
"...To align with this, we are making changes to the Membership Rewards Program Terms & Conditions that impact Card Members who are enrolled in the Membership Rewards program and are eligible for a Membership Rewards point transfer. Membership Rewards points can now be transferred
at 1:1 ratio into the Starwood Preferred Guest program and can be transferred in increments of 1,000 points"
The bolding is mine, of course, but since it transferred into your SPG program it should still be a "starpoint" until the program conversion. The actual change to the T&C is that effective 8/1 they're just straight deleting the bullet point that states it's a 1000:333 conversion rate. So anything transferred into SPG after 8/1 will transfer at the standard 1:1 rate. (or as they put it, "For most frequent customer programs, 1,000 Membership Rewards points will equal 1,000 miles, points or credits and must be transferred in 1,000-point increments unless otherwise noted." SPG would not be otherwise noted anymore)
Until 8/18 it is the SPG program and your member account number and everything stays the same.
I can't imagine they'd stop transfers to the program altogether for that time period, that seems like some kind of breach even if it might not be enforceable. If they were smart they'd figure out how to delay the transfer so that it worked in the spirit of the change. But how many folks read every single agreement change,
and collect SPG points,
and know about the program change "delay",
and would transfer beaucoup points for it? It's possible AmEx just let's it go to keep the members happy and figure out how to mollify Starriott for the fallout. (AmEx is very a win some/lose some company, IMO. Only reason I think folks stick with them is for these type of win some situations

)
But who knows? I guess we'll find out next week, but for right now it seems possible (maybe

).