An OS update really shouldn't mess with any personal data, unless you're using Nokia Care Suite, which is the equivalent of a hard-reset. Both the official Zune method and SevenEighter use .cab files in order to install the required files for the OS to function properly. Of course you can also install these .cab files yourself, but this method is much more risky, as you need to find each .cab file containing the languages packs for your handset, as well as the correct .cab's containing the OS and any firmware updates for your phone model and serial number. (i.e. the Lumia 710 has two model numbers: RM-803 and RM-809, the latter being the designate for those particular handsets on T-Mobile.)
Originally Posted by Victor Frost
All in all, it depends on you. Do you want to sacrifice the speediness of WP7.5 in exchange for a more customizable homescreen, Bing images as your wallpaper and the ability to set Google as the default IE search engine?