Is there a penalty associated with modifying an economy flight to business class on the same flight?

I'm travelling for work but our policy is economy flights only: for expensing, I have booked a return economy class ticket, with the intent of switching to business and paying the difference out of pocket. If I search via the website for business class on the same flights. Starting with the outbound leg, I can find that there's a "Royal Silk Standard" ticket available for 44k (and the most expensive is a "Royal Silk Full Flex" for 69k). However, if I modify my booked outbound flight and choose business class on the same flight, the price for the new flight is 85k. Why is a business class ticket 2x as expensive as the cheapest available ticket, and 50% more expensive than the most expensive ticket, when modifying my flight? That's just one leg, the same happens with the return leg, leading to the cost of switching from economy to business being more expensive than buying a brand new business class return on the same flights.

Recap:

1. I have an economy class (return ticket, each leg is ~22k (45k total))
2. I can find standard business class tickets available on the same flights for ~50k each leg (100k total) or full flex for ~60k each leg (120k total)
3. If I modify my booking, choosing business class on the same flight, the price is listed as 160k total, leading to a payment of the difference of 110k

I typically book business class flights on Thai Airways so I'm unfamiliar with the process of modifying a flight from economy to business: where am I going wrong?

Thank you