To save several days spent drilling out rivets, we decided to leave the tin ceiling in place. There was already a modicum of insulation behind it and it would provide a good adhesive surface for the insulation we planned to install. Height limitations made 1/2" rigid foam insulation seem the best choice and while only providing a R-3 insulative value, the existing ceiling, vapor barrier and Masonite panels we planned to install would significantly increase that number. Just like dressing for winter, every little bit helps.
Prep was quick and easy, we pulled all the light housings and speakers, leaving the wires to dangle so they could later be pulled through the new ceiling.