diff --git a/examples/playerdataconvert/main.go b/examples/playerdataconvert/main.go
new file mode 100644
index 0000000..1d7f12d
--- /dev/null
+++ b/examples/playerdataconvert/main.go
@@ -0,0 +1,199 @@
+// playerdataconvert is a program to convert player data form offline server to online server.
+//
+// When a player with official account login connect to a offline-mode server,
+// the server store the player data with their "offline UUID". While you open
+// the online-mode switch, the player data loose.
+//
+// By using this tool, you can convert the offline data into online data.
+// The players will keep everything they got, yay!
+package main
+
+import (
+	"compress/gzip"
+	"encoding/binary"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io/fs"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"github.com/Tnze/go-mc/nbt"
+	"github.com/Tnze/go-mc/nbt/dynbt"
+	"github.com/google/uuid"
+)
+
+var savePath = flag.String("save", "The save folder with \"usercache.json\" file inside", "")
+
+func main() {
+	flag.Parse()
+	save, err := os.ReadDir(*savePath)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to open dir: %v", err)
+		return
+	}
+
+	var usercache fs.DirEntry
+	for i := range save {
+		name := save[i].Name()
+		if name == "usercache.json" && !save[i].IsDir() {
+			usercache = save[i]
+		}
+	}
+	if usercache == nil {
+		fmt.Fprintf(os.Stderr, "usercache.json not found")
+		return
+	}
+	usercaches := readUsercache(filepath.Join(*savePath, usercache.Name()))
+	fmt.Printf("Successfully reading usercache\n")
+	readPlayerdata(filepath.Join(*savePath, "world", "playerdata"), usercaches)
+}
+
+type UserCache struct {
+	Name      string `json:"name"`
+	UUID      string `json:"uuid"`
+	ExpiresOn string `json:"expiresOn"`
+}
+
+func readUsercache(path string) []UserCache {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to read usercache file: %v\n", err)
+		return nil
+	}
+
+	var usercache []UserCache
+	err = json.Unmarshal(data, &usercache)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to parse usercache file: %v\n", err)
+		return nil
+	}
+	return usercache
+}
+
+func readPlayerdata(dir string, users []UserCache) {
+	for _, user := range users {
+		nbtdata, err := readNbtData(dir, &user)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to read %s's nbt data\n", user.Name)
+			continue
+		}
+
+		// Get old UUID
+		uuidInts := nbtdata.Get("UUID").IntArray()
+		uuidBytes, err := intArrayToUUID(uuidInts)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to read %s's UUID\n", user.Name)
+			continue
+		}
+
+		if ver := uuidBytes.Version(); ver != 3 { // v3 is for offline players
+			fmt.Printf("Ignoring UUID: %v version: %d\n", uuidBytes, ver)
+			continue
+		}
+
+		// Get new UUID
+		name, id, err := usernameToUUID(user.Name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Unable to fetch username for %s from Mojang server: %v\n", user.Name, err)
+			continue
+		}
+
+		fmt.Printf("[%s] %v -> %v\n", name, uuidBytes, id)
+
+		// Update UUID
+		ints := uuidToIntArray(id)
+		nbtdata.Set("UUID", dynbt.NewIntArray(ints[:]))
+
+		// Create new .dat file
+		err = writeNbtData(dir, id.String(), &nbtdata)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Unable to write %s's .dat file: %v\n", name, err)
+			continue
+		}
+	}
+}
+
+func readNbtData(dir string, user *UserCache) (dynbt.Value, error) {
+	file, err := os.Open(filepath.Join(dir, user.UUID+".dat"))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to read %s's userdata: %v\n", user.Name, err)
+	}
+	defer file.Close()
+
+	r, err := gzip.NewReader(file)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to decompress %s's userdata: %v\n", user.Name, err)
+	}
+
+	var nbtdata dynbt.Value
+	_, err = nbt.NewDecoder(r).Decode(&nbtdata)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to parse %s's userdata: %v\n", user.Name, err)
+	}
+	return nbtdata, nil
+}
+
+func writeNbtData(dir string, id string, nbtdata *dynbt.Value) error {
+	newDatFilePath := filepath.Join(dir, id+".dat")
+	file, err := os.Create(newDatFilePath)
+	if err != nil {
+		return err
+	}
+
+	w := gzip.NewWriter(file)
+	err = nbt.NewEncoder(w).Encode(&nbtdata, "")
+	if err != nil {
+		return err
+	}
+
+	err = w.Close()
+	if err != nil {
+		return err
+	}
+
+	err = file.Close()
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func usernameToUUID(name string) (string, uuid.UUID, error) {
+	var id uuid.UUID
+	resp, err := http.Get("https://api.mojang.com/users/profiles/minecraft/" + name)
+	if err != nil {
+		return "", id, err
+	}
+
+	var body struct {
+		Name string `json:"name"`
+		ID   string `json:"id"`
+	}
+	err = json.NewDecoder(resp.Body).Decode(&body)
+	if err != nil {
+		return "", id, err
+	}
+
+	id, err = uuid.Parse(body.ID)
+	return body.Name, id, err
+}
+
+func intArrayToUUID(uuidInts []int32) (id uuid.UUID, err error) {
+	if uuidLen := len(uuidInts); uuidLen != 4 {
+		err = fmt.Errorf("invalid UUID len: %d * int32", uuidLen)
+		return
+	}
+	for i, v := range uuidInts {
+		binary.BigEndian.PutUint32(id[i*4:], uint32(v))
+	}
+	return
+}
+
+func uuidToIntArray(id uuid.UUID) (ints [4]int32) {
+	for i := range ints {
+		ints[i] = int32(binary.BigEndian.Uint32(id[i*4:]))
+	}
+	return
+}