Updating the pricing of your Services will immediately update the price of each Service for all Clients moving forward. You cannot specify which Clients receive which Service cost based on the Client alone, but you have a few options to accomplish this same goal:
1. You can enroll your grandfathered Clients into a Membership program, which will charge them a set rate monthly in exchange for Service credits. This is an awesome option for ensuring repeatable income, and it incentivizes your Clients to enroll if they know your rates have increased. You can learn all about Memberships here: How to Sell, Book, and Manage Membership Plans (Video)
2. You can create an offline Service that you schedule directly for these clients. To do this, you'd simply create the Service, price it accordingly, and deselect the checkbox for "Online". This will allow you to book it while hiding it from public display. Clients would not be able to book their appointments online though.
3. You can create a Promotion using the newly increased rate, discount it by a $ or % amount so that the purchase price is the grandfathered rate. On your "Summary" page, you'll choose not to display it publicly for Clients to see, so that only those with the Promotion's hyperlink will be able to book the grandfathered rate. Here's a video that will walk you through how to accomplish this: How to Create a Promotion for a Service (Video) Then, if you wish to email your grandfathered Clients about the special rate, you can do so directly from the Promotions dashboard: How to Share a Promotion With an Email Campaign (Video).