Traveling from cold region to warm region; tips regarding clothing?

If you're not going to regions like Nepal and Northern India, don't even think to take warm clothing with you. That's from my experience. I'm travelling for a three months in Asia and I arrived here from Russia, and the only place you can possibly get cold is heavily air-conditioned transport. You may feel quite cold in sleeper buses and trains in Thailand. But all what you will need in this case is a thin fleece shirt or blanket. One time I saw some guy used his sleeping bag as a blanket -- it's really clever option for light backpacking too.

I'd recommend you to go to the airport with your friend or relative. In airport you can change the clothing and give all the winter stuff to your buddy to get it back in town.