IDBVersionChangeEvent.oldVersion

const dbName = "sampleDB";
const dbVersion = 2;
const request = indexedDB.open(dbName, dbVersion);
request.onupgradeneeded = (e) => {
  const db = request.result;
  if (e.oldVersion < 1) {
    db.createObjectStore("store1");
  }
  if (e.oldVersion < 2) {
    db.deleteObjectStore("store1");
    db.createObjectStore("store2");
  }
  // version < 3, 4... について同様に
};