Hmmmm... then you were very lucky because gospel around here is you can never downgrade the Motorola bootloader.

If you could downgrade bootloader, then everyone would go back to version where Sunshine worked! First Sunshine worked on Lollipop SU4TL-44 (November 2015), then everyone who procrastinated rushed to take SU4TL-49 (May 2016), hoping it was Marshmallow. Nope, it was just a Verizon "security" update to stop Sunshine from working. They had SIX MONTHS to unlock their bootloader and they didn't do it! Then everyone complained and said if only Sunshine worked on "49", they wouldn't procrastinate anymore. BOOM. A few months later (july 2016) Sunshine found another exploit and it works also on Lollipop "49".
But we are STILL seeing people now taking the Marshmallow OTA (November 2016) without unlocking bootloader first.

Not you, you said you bought it with it already installed...
One of the Sunshine team members said he briefly glanced at the 6.0.1 OTA and it might not be impossible to unlock the bootloader. But no promises and no ETA. I don't think this phone is a high priority anymore. This is now a 2-year old phone. Still GREAT, but other newer phones get attention. There's been Sunshine for it since November 2015 and most everyone who wants it has had a chance to get it.