Is there a one big junction box where I can just connect them all to the extensions or do I place 6-7 smaller ones?

This is usually the part where I tell you you are a fool and if you have to ask, then you already have your answer. But a 200 Amp service change out is no small feat, and anybody who can do one single-handedly has my respect...so with that in mind, I am going to give you the technical answer instead that any other electrician could understand, so I hope you can keep up. but before I do, I have some personal, friendly advice for you that if you had known before you asked this question, you never would have had reason to ask.

Bribe a friendly master electrician to grace your panel with his presence for the promise of beer, pizza, and cash to cover his gas (offer him all 3. His time is worth money and you are getting it for gas and a cheap meal). Let him poke around and "give his recommendations" because safety and because you want your system to always keep working...

HE looks at this stuff all day, and HE can tell you whats what. Thats what he does. He does not fix things (Most Master Electricians dont turn screws) and will not fix anything for a twenty and a 40 and a Little Cesar's. But he can tell you what you need to do.

The Technical answer is as follows...

You dont remove and replace a 200 Amp service if you installed it correctly; a correctly installed 200 Amp service should not ever need to be removed as a matter of location "preference. Where you put is is where it should stay until you replace it one day with a larger service.

What you can do is sub it out to a smaller service. Determine the current requirements of the extras you want to add, put all of them all inside of their own properly rated dedicated service panel, and connect the 2 panels with a disconnect in between just like any other sub-panel, and Bob's your uncle. I am sure I dont need to tell you to keep everything isolated, to keep all raceways within boxfill requirements, or to follow ALL NEC CODES?

But, and I say again, if you will just bribe a qualified Licensed Master Electrician to just stop by real quick and put his eye on things real quick and give you his opinion on his way back out the door, the lives, property, money and time you save will be your own and it wont even cost you as much as 1 single 15Amp SquareD breaker...