If it's ext2 already, upgrading to ext3 isn't really the way to go. I don't think the mechanism to do so really works well in the long run. In my tests, while I could do the process, I wasn't ever able to get disk checking to work properly. So what's the point of the journaling filesystem if you can't run a check? 
Best to start off from scratch with ext3 rather than upgrading to it.
The point of running a journaling file system is the additional security it offers ...
- let's say you open a file and modify it, when you save that modified file, the changes to the file system that are going to be made, are written to the journal before the file itself is saved (ie before the changes are made to the file system itself), if the system were to crash or a power outage were to happen in the split second between your clicking save and the file actually being written to the disk, chances are quite good that, when power is restored, those changes can be completed, from the information written to the journal, or if it was incomplete, the changes reversed or backed out, again based on the information stored in the journal - this is of course a very simplified explanation.
In an ideal situation - you'd have UPS systems to provide power protection and systems would never crash, but the real world is far from ideal and a journalling file system could mean the difference between a Point of Sale transaction that simply needs to be re-entered or the entire Point of Sale & Inventory database corrupted.
On a SOHO NAS such as this a journalling file system could mean the difference between losing all your stored data in the event of a power failure whilst writing to the disk and just running a disk scan and fixing the problem.
Also - for what it's worth - I have been able to use that disk scan tool on both ext2 & ext3 file systems - it's just not very informative as to what it is doing, has done, or has failed to do.