Ryan - I think you may have posted this same question to the GNS480 Yahoo discussion forum? If so, you received a good reply there.
Yes this is a common problem - the memory backup battery only lasts so long. Replacement of the battery is definitely within the realm of the savvy DYI'er. Check the archives on the yahoo discussion forum - there's LOTS of detailed info there, including an excellent write-up with photos.
Good luck with getting this replaced. Gotta keep those 480's soldiering on!