Start by reading the firmware release notes, those will tell you whether or not a reformat will be required.
Prior to v1.03 you had a choice of ext2 or ext3 file systems, support for ext3 was removed in v1.03 - if you were using the ext3 file system the upgrade to 1.03 would have required a re-format, if you were using ext2, it would not have been necessary.
There was also a disk partition structure change made with the introduction of v1.05 (if memory serves correctly, you can verify this from the release notes), so an upgrade from 1.04 or earlier to 1.05 or later will also require a re-format.
Whether or not a re-format is required with 1.08 will depend on the changes made in 1.08, as an example, other devices in the DNS-3x3 line up now support the ext3 file system, so it may be reintroduced - if this happens, and if you choose to use it then you will need to re-format, if it is reintroduced and you choose to stay with ext2 then a reformat will probably not be required.
The only way to know is - to read the release notes.