I managed to do the Crossflash TP-Link TD-W8980 to TD-W9980 as described in this video tutorial on Youtube. https://www.youtube.com/watch?v=vjSLGEkZd_E&feature=youtu.be
May I install OpenWrt on my ex-W8980 via the Firmware Upgrade menu of the TP-Link or I need to use the serial interface again?
... via the Firmware Upgrade menu of the TP-Link
Not listed as an option according to OpenWrt: devicepage
With the TD-W9980 firmware, the web interface hack should work. It does not require opening the case.