Well searching through the forums i've found that ocassionally people with various products performing upgrades encounter some sort of problem, but this is a small percentage in the grand scheme of things.
I've got a handful of Terastations and Linkstations, and have -never- had an upgrade problem of any sort....although in most cases i did backup the data first as a precaution.
So backup your data and away you go. The firmware update is 'not supposed' to remove any of your data.
Keep in mind if you don't have a reason to update (in this case, if you're not using a PS3, or not wanting Web Access upload) then I'd say don't upgrade, but if you require either of those, sounds like you don't have much of a choice. Backup and go, making sure you follow appropriate procedures. Also make sure you validate the integrity of your backups so you know you have them as a fallback.