I think I would be more wary of the agency after hearing of this substitution. Naturally, all tour operators have the right to substitute "comparable" alternatives, but I think a reasonable person would not consider THAI and Tiger to be comparable. Yes, you could try to work something with the agency based on your expectation that you would have 20kg of luggage, but I would be a bit more concerned that this might represent a bit of misrepresentation on the part of the tour operator.

If you haven't yet left on the tour, it might be interesting to contact the agency and see if there is any refund for the obvious downgrading of the intra-Asian air transport.