Three points.

i) If it is a Multi City trip or a person wants to travel the same route with one way in Economy and the other in Business, then the system automatically chooses the cheapest fare Class. In this case choose Pay Later, send booking ref to Ticketing Office and request to issue ticket with a higher Booking Class.

ii) Many times I have changed Booking Class after issue, the reason being that I want the option to buy a standby upgrade and the overall cost is cheaper by changing Booking Class before (and Client pays for it)

iii) Excess baggage can be paid for. I never have to worry about baggage allowance due to status and travelling light unless carrying my wife's shopping, but historically Thai have not been as strict as other airlines with baggage allowance. I have observed Thai being more strict at a couple of airports outside Thailand in recent months.

Apart from above one option is to cancel ticket and obtain a refund, then book again.