Don’t do it online! The space will be assigned to any elites in waiting lists before it goes back to inventory.
do this over the phone. The agents are competent to handle this.
Explain what you want to do, confirm they understand the two step transaction you have requested (cancel and rebook). Then confirm they see award availability on Tokyo-Bangkok, then tell them to execute it.
All the explaining and confirming is to enable it be caught in voice recording and thus give you some negotiating power in case of a screwup.