Not sure why would you want to swap those buttons but you can certainly do it using [AutoInput](https://play.google.com/store/apps/details?id=com.joaomgcd.autoinput) (for Android 4.3 and above), [Tasker](https://play.google.com/store/apps/details?id=net.dinglisch.android.taskerm), both are/have paid/7-day full trial, [MacroDroid](https://play.google.com/store/apps/details?id=com.arlosoft.macrodroid) (free/paid) and [Shortcut Launcher](https://play.google.com/store/apps/details?id=com.mibi.shortcutlauncher)/ [AutoShortcut](https://play.google.com/store/apps/details?id=com.joaomgcd.autoshortcut&hl=en) (free/paid).
1. After installing **Shortcut Launcher** you would see four icons namely **Shortcut 1**, **Shortcut 2**, and so on in your app drawer.
Tap **Shortcut 1 → MacroDroid Shortcut → `<Add New Macro>` → Volume Up/Down → Volume Down** → proceed → ignore **Add Constraints** → name it "`Volume Down`".
2. Similarly, assign a MacrDroid shortcut to **Shortcut 2** but choose **Volume Up** and name it "`Volume Up`".
Alternatively, AutoShortcut can be used in place of Shortcut Launcher. Use it in a Tasker's task to directly access a MacroDroid shortcut. It is available under **Plugin**.
3. In Tasker create an **Event** based profile from **Plugin → AutoInput → Key → Configuration**:
* **Keys**: **Volume Up**
* **Key Action**: **Key Down**
4. Create a task and add following actions:
1. **Plugin → AutoInput → Modes**:
* **Configuration**:
* **Key Suppress**: **Enable**
* **Keys: Volume Up**
* **Timeout: None**
2. **App → Launch App → Shortcut 1**
5. Create another **Event** based profile but for **Volume Down** (follow *step 3*)
6. Repeat *step 4* but under *sub-step 1* do **Keys: Volume Down** and assign the app **Shortcut 2** in *sub-step 2* there.
That's it. It's not completely flawless but the flaws can be ignored.
For any reasoning about related to AutoInput see my answer at [Bind action to hardware key combination](https://android.stackexchange.com/a/123018/96277).