Yea, I figure I'll just have to brute-force it (backup, repartition, restore, edit fstab). I suppose another option would be to convert back to ext2 and see what Partition Magic can do, then convert to ext3 again when finished (but that may be more trouble than it's worth).
Anyway... I already have backups of both my Windows & Linux drives, so I won't lose anything if something goes wrong. I did a full partition backup last week when I tried to get Linux running on an Intel RAID-0 array. The Linux part went okay, and wasn't even all that difficult. I had Mint booting off of the RAID volume with home in its original partition on my 80G drive. Grub wouldn't boot into my Windows 2000 install, though. Error-15 if I remember right. Googled around and tried everything suggested and just couldn't get Windows booting. Finally gave up and did a partition restore and went back to using the BIOS to change the boot order.
So today I was getting ready to just get rid of the RAID volume and run the drives individually, when I started wondering if something might have been wrong with the MBR of the RAID stripe... Partition Magic couldn't read it (bad MBR), but all of my other software could. At first that made me think it was just a PM issue, but after the grub problems I wasn't so sure.
Since I had partition backups of my 2 boot drives, I decided to just blow the RAID volume partitions away completely and use Partition Magic to set them up again. That went well. I set up my Windows boot partition, along with /, /usr, /var and swap for Linux, and 2 more NTFS data volumes to use up the rest of the space. Then I restored my Windows boot partition from the backup and booted my Mint live CD. From the live CD I reinstalled Mint to the RAID volume (using the dmraid software), and after going through the little contortions needed to get grub installed on the RAID volume I rebooted, and it WORKS!
I can now dual-boot Mint & Win2K with grub from my Intel SATA RAID volume.
I'm probably still going to get rid of the RAID, but I'm gonna keep it for a week or so and see if I can really notice a worthwhile performance boost. First impression is that boot times are faster, but not dramatically so. Shutdown, however, is super-fast!
Thanks again to everyone helping out here! I have learned a great deal and gained a lot of confidence in Linux, and the helpful people here are largely responsible for that. This really is a great community.