Update A Hidden Field From Post_Submission Hook

Need to update a hidden field in a form with an ID number after the form has just been submitted. The ID is calculated by PHP running in a gform_post_submission hook within the theme's functions.php file.

I thought about writing directly into the GF database like this:

$wpdb->query("UPDATE $wpdb->wp_rg_lead_detail SET value = 'New Value' WHERE lead_id = 30 AND field_number = 81");

However, that doesn't seem to update. No error given, just no update happens.

Is there a smarter way to do this by calling a function already in the Gravity Forms library?

Good question. I'm using the form to collect customer info including credit card billing that is sent to Authorize.net for payment processing. When the processing transaction result is returned by them via XML message, they send back the customer's ID they generated. I'm writing the new customer ID back into the Gravity Forms database. I also need to write the payment status (OK, declined) back into GF plus I want to overwrite the customer's credit card # in the GF form database so only the last 4 digits remain. (I'll let Authorize store that risky info so I don't have to implement lots of security on my end.)

Given that workflow what do you recommend?