A 3200XM can run 6.3.1.2. The installation requires 40MB free memory if upgrading from the CLI or 60MB from the WebUI. In your case, you may need to update it via the CLI. You may also need to free up some flash space and possibly reboot the controller prior to update to free up some memory. If you reboot to free up memory, upgrade immediately.
Check the Memory Requirements of the attached release notes.
Also, note that thd 3200XM will start to lose a few of 6.3 features as indicated below: