Help adjusting timestamp on existing nodes

I have about 30,000 existing nodes with their date field (field_event_start) configured as "Site's time zone".
I had mistakenly thought this was storing values in the database in my site's time zone - however, I now understand that they are stored in UTC (+8 hours).
I want to change my configuration for this date field to be "No time zone conversion" but I will need to adjust all the existing values first.
I'm hoping someone with better MySQL skills can provide me the steps to adjust the "field_event_start_value" and "field_event_start_value2" fields (both are varchar).
Logic:
For all rows in table "content_field_event_start"
subtract 8 hours from the current value (obviously the day will need to change accordingly, not just the hour)