Auto Loan Rates Discounts- "Engagement Score"

When it comes to auto loan rates, we have certain rate discounts that may apply based on an "Engagement Score" for each person. This is determined by our underwriting team and communicated to the dealer for the best rate available at the time the deal is happening so that the correct rate is listed on the contract. The problem is after 5pm when our underwriters go home, we are still getting deals since dealerships are open later. The next day we have a contract signed for the rate the customer qualified for but does not include any engagement score discount. My question is: After booking the loan at the rate listed on the contract, may we then adjust the rate down to include the engagement score discount the customer should have received without having to ask the member to sign anything? I assume we could adjust a rate like this so long as it benefits the customer and we document what we're doing and why, but I'm struggling to find a definitive answer in a Reg.