If you have sold Service Certificates for a certain amount prior to raising your prices, the value of the gift certificate will not update with the pricing update. Now that your prices have increased, recipients of the Service Certificates will no longer be able to apply them in full for the Service and instead will be presented with a balance equal to your price increase. This is the correct behavior for Service Certificates, as ultimately, they are good for a certain dollar amount versus a certain service.
There are a few solutions here that may help you honor these certificates for the service, and not just the cost:
Apply Discount at Checkout
If the client will be redeeming the gift certificate code at the time of their appointment, you can apply a discount during checkout that discounts the Total Balance Due down to the original price of the Gift Certificate. For example, if the gift certificate was sold at $100 and your price has now increased to $120, you would want to apply a $20 discount at the time of checkout so that the $100 gift certificate covers the total balance due: Step 6 of Checking Out an Appointment.
Add Store Credit
If the purchasers are the ones who will be redeeming the gift certificates, or if you know who is going to be redeeming the gift certificate, you can add Store Credit to their account under the Client List > Client Record > Account tab. Store Credit shows clearly to the client during online booking as a payment method. So when they see the balance due at checkout, you and the client will have the option to apply the Store Credit to pay the balance: Adding Store Credit and Applying Store Credit During Checkout
Enable Partial Prepayment
You can enable partial pre-payment under your Settings > Online Booking Rules > Prepayment Rules, allowing the client to redeem the Gift Certificate, and when they come in for the appointment, the remaining balance can be discounted in the Cash Register by you: Activating Pre-payment and Deposit Requirements and Step 6 of Checking Out an Appointment.