How do i make my new TimeCapsule the primary router for an existing network and continue to support my AirportExtreme extensions?

I think you have it correct. With the Time Capsule, you should no longer need the Airport Extreme. However, you'll have to set up a new Time Capsule wireless network. It's not any more difficult than it was on the Airport Extreme. I think if you even give it the same network name and password, all of your devices should connect seamlessly. If not, then you'll have to reconnect the various devices but again, a very simple process. One thing to know though is that the "out of the box" recommendation is to set the Time Capsule to bridge mode. (It's a one-click setting in the Airport Utility.) I can explain more what that means, but if all you're trying to do is use the 802.11ac wifi speeds and have one wifi access point, then bridge mode works without a hitch.

Again, I don't think you'll need or want the Airport Extreme. (Hello, craigslist.) I would try just the TC to see if everything starts working well. Cluttering the airwaves with two wifi access points can be tricky, so best to avoid if you don't need it.