How do I add Rise of Tiamat to my existing Horde of the Dragon Queen Game?

I've got an active game and used the HotDQ module but now we need to add the RoT content. I've bough both modules - do I need to start a new game and port everything over using Character Vault or can I just add the module?

It's an entire campaign so you have to start a new game

As Ed said it is a module not an addon so you would have to create a new game with it but as you are a pro, you can use the transmogrifier to shift anything custom from your existing game over to the new game so there would be an almost seamless transition.

Transmogrifying would take forever. Starting a new game would be more efficient if you didn’t add many assets to Hodtq

I think Pat meant to start a new game with RoT, and just transmogrify over to HotDQ any unique character sheets or custom handouts that were made during the course of play.

Wait, do you mean assets I've bought for from the community content in HotDQ won't be available in RoT?

They absolutely can be available. But you will have to move them in. A game that contained all of the pages, handouts, characters and tokens of both modules at the same time would likely be very unmanageable. You can use the transmogrifier to move anything between those campaigns of course. If you are a free or plus subscriber, you can vault characters around. So yes, you have everything, just not all starting in the same game by default.