@Alan ****
Would it be any cheaper?
For simplicity, Thai Airways ticket one way Tokyo - Bangkok is 10,000 THB (return is 20,000THB).
Return as one way is also 10,000 THB.
Those tickets are the cheapest. Any change of dates - 200 US$ fee.
Flexible tickets (dates change) are 40-50% more expensive upfront.
Edit: say, you are entering on 60days visa exemtion or on Non-O 90days. Then decide to xtend for 30 more days. You are in for that fee as you would have to change your return date.
The way around it may be to buy the initial ticket as the cheapest one, Then buy, at the same time the more expensive, independent one way return ticket with a flexible date.
If you buy them as return at the same time, one transaction, the onward the chepaest, the return flexible, the rules of the cheapest one apply to return ticket as well.