I have a 2011 Atima with 198k+ miles. I want to find out if the transmission has ever been changed to new one before I purchased the car. How can I find this out?

If it was done under warranty or goodwill then a dealer can tell you by simply running the car's service history. If not, then I don't think there's any good way to tell. Nissan PCC would have a record you could access via the QR code on the Range Switch or the casing numbers, but PCC is a PITA and I doubt most dealers would be willing to do that for you without being paid for the effort. You could try inspecting for tool marks on fasteners that would only come loose if the tranny was being split from the engine.