***EDIT*** 2015/05/21 @ 12pm PST
How about some good news? THIS IS RESOLVED!
We've completed the update path building, device testing, and the ability to update is now in place. We just launched this about 15 minutes ago.
- Double-check you're registered to Insider Fast
- Reboot your device
- Check for updates
You should now be able to update to Win10. :)
I know the wait was difficult, but hopefully it will have been worth it. If you hit any issues in the update process, please post back.
If you're reading this thread, you're likely on OS version 8.10.15127.138 or 8.10.15135.146. If that's right, please keep reading.
Explanation on this one will be a little lengthy, so please bear with me.
To get from WP8 up to Win10, there has to be a software update path created. When an update is made available on the server, your on-device OS checks to see where you are currently (what version it has), where you'll be going (what is new on the server), and it validates that there is a valid path between the two. To over-simplify it, think of being in city A (WP 8.1), wanting to go to city B (Win10), and having a map that shows your device how to get there (the update path).
The issue for users hitting the current "no build" situation... is that there's no map in place. The software update path from your current OS build up to the new Win10 build is not in place yet.
There are a few different variants of the 640 device. Those who have devices that are on the very newest software (the versions I listed at the very top) are the ones that can't update to Win10 yet. It's unfortunate, we know about it, and we're working to build in the update path to make this available for you. It's going to take a bit of work to complete this.
I know: It's a bummer to hear "hey, your phone is supported!" only to turn around and hear "but, we found out that some of you still can't update". There isn't much that we can do to make up for that disappointment at the moment. The only consolation is to be open with you all as Insiders and share the situation at hand.
We'll keep you posted for when we have the right update path in place for you. It'll probably be early next week. We'd rather you know that instead of keeping you waiting day-to-day. Hopefully this bit of transparency and insight is helpful.