Instead of (Un)marshalling into a valid struct, when you only care only about one field (in perhaps a dynamic JSON document) you could do the following

change-raw-JSON.go

package main

import "fmt"
import "encoding/json"

const data = `{"a": "A", "b": "B", "c": "C"}`

func main() {
	var objmap map[string]*json.RawMessage
	err := json.Unmarshal([]byte(data), &objmap)

	if err != nil {
		fmt.Println(err)
	}	
	
	objmap["a"].UnmarshalJSON(
		[]byte(`"FOO"`),
	)
	fmt.Printf("a now is: %s\n", string(*objmap["a"]))

	out, err := json.Marshal(objmap)
	fmt.Println(string(out), err)
}

/* Output:
a now is: "FOO"
{"a":"FOO","b":"B","c":"C"} <nil>
*/