Oneplus TWO A2003: convert OxygenOS 3 Beta to HydrogenOS (ALL STOCK)

Hi guys,
my Oneplus TWO is totally stock (no ROOT, nor Recovery, locked bootloader).
I just want to ask how could I "convert" it to original HydrogenOS, with working OTA feature.

Is there any procedure to follow?Is it possible or I need to unlock IT and flash a custom HydrogenOS firmware?

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