iPhone 13 and iPhone 14 look very similar, and the specifications are very close in size and size, so it is possible that the two devices share a case. The iPhone 14 is slightly thicker than its predecessor, but the overall difference is so small that the iPhone 13 case may be enough for the capabilities of your new iPhone 14 model. Some merchants are already selling universal case for iPhone 13 and iPhone 14, so if you are migrating from iPhone 13 to iPhone 14, you can keep the old case. Of course, if you don't feel the right level is good enough, you can always change the latest phone case.
However, there are significant differences in hardware for iPhone 13 pro, iPhone 13 Pro Max and iPhone 14 Pro, iPhone 14 Pro Max or iPhone 14 Plus models, and it is completely impossible to use the mobile phone case above iPhone 13 pro, because iPhone 14 Pro, iPhone 14 Pro Max has a larger camera and a new button position.
So in fact, only iPhone 14 can use the previous iPhone 13 phone case, other models need to buy new phone cases.