I think some phones have gotten the firmware bump in January and February...
Check against Windows Device Recovery Tool if you are still on 8.1 and see if there is a Firmware increase available to you. I got one in January from WDRT for my 1520.3. I've read on the forums of a few models getting Firmware since 2016 started.
On moving to 10 official I got zero increase in Firmware version number.
If you are on Insider already, ask in your device forum for an 8.1 user who is up to date to share their firmware version number with you for comparison.
Theory: Firmwares were prepared earlier and the build was waiting ti stabilize before release to teh general user public. Firmware was pushed when ready.
YMMV