Problem - When I went to customize my Spartan Service Tag for the first time in game, after entering in the new 4 characters in the letter picker screen and choosing Done, the tag did not show as changed when returning to the Customization screen. I repeatedly tried different tags, and never received an error, but the tag never changed from my starting 4 letters.
Resolution - Rebooting the console and reloading H5 showed the last Service Tag I had entered, and all attempts I have made since then to change my Service Tag show the updated new tag in the U.I. immediately after the change as expected.