I have found my pet sitter but before I book I would like to change my request from day care to boarding and I am not seeing that option.
There is no way to change it or to even delete it. What you should do is create a new request for boarding and proceed. You will NOT be charged for two services since there are procedural steps that prevent it. The sitter officially ACCEPTS the booking and then it returns to you to confirm payment. As you can see, you would need to confirm the service you need and would NOT do so for the other.
Unfortunately, Rover's system is garbage, lacking normal features to help pet owners and sitters.