package main

import (
    "errors"
    "fmt"
    "log"
    "reflect"
    "time"
)

type User struct {
    ID       int    `json:"id"`
    Username string `json:"username"`
    About    string `json:"about"`
    IsAdmin  bool   `json:"is_admin"`
    Status   int    `json:"status"`
    Date     *time.Time
}

func main() {
    now := time.Now()
    ua := User{
        ID:       1,
        Username: "admin",
        About:    "I am an admin",
        IsAdmin:  true,
        Status:   1,
        Date:     &now,
    }
    ub := User{
        Username: "user",
    }

    fmt.Printf("ua: %+v\n", ua)
    fmt.Printf("ub: %+v\n", ub)

    err := Replace(ua, &ub)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("\nua: %+v\n", ua)
    fmt.Printf("ub: %+v\n", ub)
}

// IsZeroOfUnderlyingType return wether x is the is
// the zero-value of its underlying type.
func IsZeroOfUnderlyingType(x interface{}) bool {
    return reflect.DeepEqual(x, reflect.Zero(reflect.TypeOf(x)).Interface())
}

// Replace replaces all fields of struct b that have a
// zero-value with the corresponding field value from a.
// b must be a pointer to a struct.
func Replace(a, b interface{}) error {
    // Check a.
    va := reflect.ValueOf(a)
    if va.Kind() != reflect.Struct {
        return errors.New("a is not a struct")
    }
    // Check b.
    vb := reflect.ValueOf(b)
    if vb.Kind() != reflect.Ptr {
        return errors.New("b is not a pointer")
    }
    // vb is a pointer, indirect it to get the
    // underlying value, and make sure it is a struct.
    vb = vb.Elem()
    if vb.Kind() != reflect.Struct {
        return errors.New("b is not a struct")
    }
    for i := 0; i < vb.NumField(); i++ {
        field := vb.Field(i)
        if field.CanInterface() && IsZeroOfUnderlyingType(field.Interface()) {
            // This field have a zero-value.
            // Search in a for a field with the same name.
            name := vb.Type().Field(i).Name
            fa := va.FieldByName(name)
            if fa.IsValid() {
                // Field with name was found in struct a,
                // assign its value to the field in b.
                if field.CanSet() {
                    field.Set(fa)
                }
            }
        }
    }
    return nil
}

输出量

ua: {ID:1 Username:admin About:I am an admin IsAdmin:true Status:1 Date:2017-05-11 17:47:30.805657327 +0200 CEST}
ub: {ID:0 Username:user About: IsAdmin:false Status:0 Date:<nil>}

ua: {ID:1 Username:admin About:I am an admin IsAdmin:true Status:1 Date:2017-05-11 17:47:30.805657327 +0200 CEST}
ub: {ID:1 Username:user About:I am an admin IsAdmin:true Status:1 Date:2017-05-11 17:47:30.805657327 +0200 CEST}