Knowing how airlines love to screw over everyone as best as they possibly can, I, personally, would hold off on the ticket buying until the merge is 100% complete. Knowing them, you will end up at the desk & it turns out you have an ‘old’ ticket from the ‘old’ airline, which needs to be upgraded & converted into a ‘new’ airline ticket, which will come with a fee most likely. Since it’s a merger, maybe they plan on charging twice for baggage fees, once for each airline.