I'd suggest you start by reading
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^] (again)
Creating a new Service contract in a new namespace is just that - a new Service contract...
So forget about this and start with a "fresh mind". Ask yourself: Do I want strict versioning? (read MSDN article) or does lax versioning do the job? (was 100% true for all cases I had over the years). Better you read for yourself and ask for specifics you may don't understand. But some general advice - never remove anything from existing Service contracts, just add. And if message-contracts may change (unlikely if you just use them as DTO's), use interfaces instead and decouple the message contracts from implementation in the same way you'd do with Service contracts.
If you want strict versioning you have to change the clients if you have a new "strict" version, for "lax" versioning you may not change the clients as long as the new contract is "compatible" (=didn't remove anything, just add.) If you then want to use e.g. a new service method from the newer contract on client-side you have to change the clients anyway...