How do I upgrade to WM 10 if my carrier doesn't support my phone?

I'm using an eBay bought unlocked AT&T version of the 640 XL on T-Mobile. I'm running the Insider Preview of WM 10 and would like to use the normal published version of WM 10. If I reset to factory version, I go back to WM 8. T-Mobile doesn't support the phone (I don't think), so I'm guessing I won't get a notice to upgrade to WM 10.

Any options to get regular WM 10 or should I stick with the Preview?

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