I want to change the full-Access API key configuration to 'read-only' access after integration.
If you have configured Chargebee-Salesforce integration with an API key that has "full access" rights and then change the rights of this "Billing_api_key" to "read-only" for various purposes, it is currently not possible.
Once the API key is created you will only be able to delete or disable the API key, you will not be able to change the rights to read-only. In order to do that you will need to create a new API key with read-only API key rights.
Also, for Salesforce integration, you will need to use a full-access API key as mentioned in this document.