When Microsoft announced the Lumia 550, 950, and 950 XL back in October, they stated that the devices would ship in November, with Windows 10 Mobile rolling out to older devices in December. That ended up being a misstep.
Since then, the update was delayed to January, then to February, and then to March. Microsoft Mexico even stated once that the OS would roll out on February 29, which obviously didn't end up happening.
The latest rumor is that the OS will ship to older devices this Thursday, March 17. The rumor comes from WindowsBlogItalia, which has a track record for leaking Microsoft-related materials such as this.
Of course, even if the OS does ship on March 17, most devices still won't receive it. The source states that the devices to be upgraded on Thursday include: Lumia 535, 636, 638, 640, 640 XL, 830, 930, and 1520.
The list is a bit odd, as it doesn't coincide with Microsoft's list of which devices will receive the OTA first, which includes: Lumia 430, 435, 532, 535, 540, 640, 640 XL, 735, 830, and 930. These are all Windows Phone 8.1 devices; however, the Lumia 1520 is a Windows Phone 8 devices, with its appearance on the list cause for speculation.
Also, even if this rumor is true, it's not likely that everyone will receive the upgrade at the same time. Carriers do still need to approve the initial Windows 10 Mobile upgrade, even if further software updates will not require it.
Anyone who just can't wait any longer can simply get Windows 10 Mobile through the Insider Preview. The official update will not be bundled with any device specific firmware, so users can get the exact same bits through the Release Preview ring as if they decided to wait.