I have unlocked Rogers Lumia 900 and have still not received 7.8 update. I have asked couple of people using Lumia 900 on Rogers and seems like everyone got their update but me.
Updates via Zune, WPHacker's SevenEighter program, or the manual push method all involve sending .cab files containing the software and/or firmware update. Since the installation just patches the current OS, no personal data is lost in the process. Only with the flashing of custom ROMS will all data on the phone be lost, since you are basically installing the OS to your phone, not just patching it with an update.
I updated my Rogers Lumia 710 from WP 7.5 to WP 7.8 through Zune PC Software with a USB connection back in February 2013. I've had no issues since with WP 7.8.