Jordan Kahn, writing for 9 to 5 Mac:

So far Apple has only shown the Watch in pre-assembled collections, which reduces their potential for customization, but our sources say that Apple’s retail outlets will be stocked on day one with individual straps so customers can easily mix and match the looks of their Watches.

I would’ve been surprised if this wasn’t the case. But, I’d also expect that some of the bands will be in limited quantities or will only be available in-box with the more expensive Apple Watch models.