There should not be a problem with switching the pairing. As Froxo says, unpair from the first phone and pair to the second phone. The only disadvantage is that it takes a little time to complete so I will make the obvious suggestion, simply leave the watch paired to your preferred phone and swap the iPhone's SIM to the one appropriate for where you are.
There is one caution about re-pairing on different iPhones, and that is Apple ID and Activation Lock. See About Activation Lock on your Apple Watch - Apple Support for a guide, and note that strange things may happen if you have a different Apple ID on the iPhone and the Apple Watch. Ideally you want the phone and the watch on the same Apple ID, which probably means you would want both your iPhones on the same Apple ID. (You might already have this so not a problem, just a warning about a hole into which the unwary may fall!)