Loading a new Open API JSON file as Definition will not override most query or body parameter settings on an API Request, one that is bound to an API via a Definition, and appears under the APIs tab.
Most (but not always) changes you have made to a particular parameter (such as exposing it as a driveable {{var}}) will be preserved. For a POST Request, you can even change the POST mode from say www-form-urlencoded to form-data (as required for sending files via Postman), and your change will stick.