Last time Windows Phone 8.1 GDR2 (which is now in the Windows Phone 8.15.xxxxx build series) was rumored to come October/November. I don't think Microsoft is planning to release the Windows Phone Threshold Preview with the Windows Threshold Preview. I do think, however, that Microsoft will release Windows Phone Threshold and Windows Threshold as final products arround the same time. I mean, Windows Phone updates don't need 8 months testing before it can go out to the public. So there is plenty of time to release GDR2, perhaps GDR3 and then move on to the big update Windows Phone 9 for spring next year, together with Windows 9.
On the other hand, the fact that Microsoft bumped the Windows Phone version from 8.10 to 8.15 let me think that we're getting something bigger then GDR2. Also, they aren't naming it "Windows Phone 8.1 Update 1", but just "Windows Phone 8.1 Update". They did the same thing with Windows, there have been a lot of rumors of and GDR2 for Windows, but eventualy, there was no update. Just an announcement that Microsoft would start rolling out features every month. I think Microsoft will take their time and use it to release a "Windows Phone 8.5", after all, Microsofts own software made references to such a release.