Are you sure it just wasn't a change of fare? You generally will have to pay the fare difference. And the closer to the travel date, ticket prices will usually only go up. And different days can be different prices.
Yeah, you will definitely want Baht in your pocket, no matter how you get it. Most tourists exchange their Dollar/pounds/euro etc as soon as they can after arriving. Save the credit cards for larger purchases, or emergency withdrawals.
, sealed route implies that you will not be entering Bangkok, and your luggage goes all the way to your final destination on its own. This means (most likely) not even going through immigration there. And I believe it also implies that you have your next boarding pass already. I don't specifically know the Thailand rules, but I know that in both Vancouver and Narita in November, we were escorted pretty much everywhere until we got to the departing gate area. Our luggage went to the final destination.
If you can change your flight route, I would look into that. But definitely try to keep to the same airline if you have checked luggage. That will help a lot.
, yes, I remember that happening for the previous version of test and go when I applied. It is always something I have in the back of my head when filling out forms. You'd be surprised how often you come across it.