Okay, I see what the issue is. There are a lot of roundtrip PS fare filings on TPA-BKK, so EF does not show them all if you are searching for flights further out and it doesn't make it to the M basis PS fares. The cheapest roundtrip ones with an M basis look to go for $5068 roundtrip. The min stay requirements are encoded in the 5th letter of the fare basis code (a 3 means 3 days, and a Z means 12 hours in examples below). The third letter encodes day of travel requirements.



You are potentially looking at up to around $2800 to upfare in both directions (from cheapest V basis PS fares that book into G class). If you are only looking to upgrade the KE leg in one direction, you can just upfare in one direction and not the other. With roundtrip fares, you do not actually need to book the same roundtrip fares in both directions of travel. EF fare listings show total price with the same roundtrip fare in both directions, but it's not actually a requirement. You just need to meet the roundtrip booking, min stay requirements, advance purchase requirements, etc. of the cheapest roundtrip fare to combine it with a more expensive one. But you are still looking at potentially paying up to $1400 just to upfare one direction of travel to an M basis PS fare.