The only way to accomplish this (that I know of) is to use the acf/save_post with a priority of < 10. This means that your filter will run before ACF saves any values to the DB. You would access the fields in $_POST[‘acf’]. You would use these field keys in your update_field() calls using the field key (not the field name). When you are done unset the input array
This last step will prevent ACF from saving anything because nothing exists.
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📣 “ACF Chat Fridays”— Advanced Custom Fields (@wp_acf) January 31, 2023
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