Dan,
You can use Upgrade Advisor... Most likely it will give you a "Good news, your device qualifies. We are working with your carrier," type of result.
Truth is... No one knows if or when T-Mo will approve the upgrade for an OTA push except T-Mo. Considering that they dumped the 640 after about 4 months, chances don't look all that good.
If you use Insider on the Release Ring, the same version is fetched as the recent public Upgrade received by these phones (10.0.10586.xxx): Phones Eligible for Windows 10 Mobile Official.
Since your model is on the supported list, you can avoid the waiting and wondering by using the Windows Insider App. Select the Release Ring to get the current (Threshold 2) branch and match the RTM build and version numbers. Afterward, you may use Insider again and select Production to un-enroll your phone from Insider, but at least you have W10M. WaaS (Windows as a Service) will deliver future updates within Threshold to you without any Insider access required.
Or you could stay on the Release Ring and you will get the next public version early as Release Ring in effect is the RTM. This will deliver the most stable Redstone builds to you earlier than OTA push with minimal risk.
Check out this thread: Windows Insider Testing and You [Recommended Guidelines] For right now, Windows Device Recovery Tool is only flashing 8.1 to models that did not launch with 10 so it is the way to revert if you don't care for W10M: How can I recover/reset/restore my phone software?
