I have been visiting AT&T forums for Lumia 640. No release date in sight yet. But I have tried .218 (release preview) on my phone and honestly, the good points about Windows 8.1 compared to 10 are more than enough for me to stick to 8.1 denim.
MUCH better battery life, smoother performance, a Store app that is not plain dumb when it comes to checking for and installing updates etc.