Yep, I do have a branded version, it's the AT&T GoPhone Version. This is my concern, been burned by carriers in the past and why I'm strongly considering going insider. The 635 (GoPhone variant) never got 8.1 update 2 while the contract ones did. ATT, could in theory do the same all over again, only deliver W10M to "select" 640's. I also seriously doubt the Tmobile branded 640s will ever receive the W10M update, since it was only carried by Tmobile for 2 months.
I have the 640 GoPhone variant as well. The thing is with the GoPhones, AT&T will update the regular phones, but they generally leave the GoPhone customers out in the cold. I do not expect the GoPhone 640 to get the update when AT&T updates the "regular" post-paid 640s.
I went to Windows 10 on mine several weeks ago. Updated since then to a new build, and it is running great. I can't see myself ever going back to 8.1 again. Loving 10.
HINT: If you join the Insider program to get on W10M, then after you upgrade the phone, you can open up the Insider app, tap the ellipses (three dots at the bottom right), and select "leave program." This will remove you from the Insider program and you will no longer get the Insider builds. However, when the next available Windows 10 Mobile upgrade is available for the general public, you'll get it with all the 950s, 550s, etc. It will be just like having an official upgrade, except you get it sooner.