I don't think you should need to default the interface first. If you do a PUT it should overwrite whatever is in place with what is in the PUT.
As an alternative, you could also look at using the YANG Patch media type for the RESTCONF request. A YANG Patch allows you to perform multiple actions within a single transaction in NSO, and they don't necessarily have to be the same type of action (so for example, you could delete the description and access-group from an interface and add a new BGP peer in a single RESTCONF command). It takes a little more effort to work out the correct data to send, but is a much more flexible way to implement changes.
To use a YANG Patch, you should send a PATCH request to /restconf/data with a Content-Type header of application/yang-patch+xml and an Accept header of application/yang-data+xml. Below is an example of a YANG Patch data body:
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