To the best of my recollection, NONE of the firmware updates forces or requires a reformat of the drives, with the sole exception of 1.02b, and even then, only if you were running an ext3 file system (1.02b removed support for ext3, it was reintroduced in 1.08).
With regard the addition of utf-8, I believe that was done in 1.04, and whilst it does require a format for it to work, the update does not force you to format, and if you don't need utf-8, you can simply leave the format until it's convenient.
To further re-inforce lookingforinfo's point on backing up - even though none of the firmware updates forces a reformat, a glitch during that upgrade, or for that matter, a power outage whilst writing to the disk, can corrupt your disk, at which point you can kiss your data goodbye - unless you have a back up.