At the very beginning, you should download the iPhone SMS transfer tool.
It empowers you to transfer messages between iPhones with simple steps. For example, from iOS 7 to iOS 8.
After downloading Phone Transfer on your computer, install it. After the installation, launch it and click the "Start" button.
Note: Make sure that you have installed iTunes on your computer. Otherwise, the process can't move on.
Step 2 Connect two iPhones to the PC
Connect both your iPhone to the computer with USB cable after running Phone Transfer. Once your iPhones detected, your iDevices will be shown on the interface. The source on the left and the destination on right. If they are not on the right position, click "Flip" between them to change their position.
Step 3 Transfer SMS from iOS 7 to iOS 8
If you want to transfer your text messages only, you can uncheck the other boxes and have the "text messages" left only. Then, click "Start Copy". Just wait for few seconds, the program will complete with no time. When it's finished, click "OK".
Note: Make sure that the USB cables are well connected during the transfer.
When you finished the three steps above, congratulations, you have had your iOS 7 SMS well transferred into your iOS 8. You can also transfer your contacts, photos, videos, etc. That means you can also transfer iPhone contacts to iPad. No matter which type of data you want, you just need to follow the steps above. Why not download it and have it a try. After that you don't have to ask how to transfer text messages any more.