Apple Watch Series 1 compatibility with iPhone 5c

I have an iPhone 5c (which comes with the Apple Watch app already installed) and have just purchased an Apple Watch Series 1, because I understood that an Apple Watch Series 3 does not pair with an iPhone 5c. I tried to pair my new Apple Watch Series 1 with my iPhone 5c that is running OS 10.3.3, but the Watch app on the iPhone is asking to update the iPhone's operating system ... but, after checking, I find there is not one available. Now searching again through the support site, it appears that if the Apple Watch Series 1 is running version 4.0 software it will not pair with an iPhone 5c. Is this true? There is nowhere on the Apple Watch packaging or on the Apple Watch's screen to determine the version of the software it is running. Therefore, can I assume that any new Apple Watch Series 1 is no longer compatible with an iPhone 5c?!!!

My follow-up question is ... why am I so dedicated to Apple products when upgrading the OS on the Apple Watch will make it no longer compatible with my iPhone?!