I'm not really a sysadmin expert, nor a RAID expert.
There's probably some nifty way to just retag the partition as a non-RAID filesystem, but I've never tried/done it.

Copy off the RAID5 data to the new drives first, whatever you do.

Then your old 1TB drives will all be "available" for experimentation. I would simply partition and mkfs on a new drive (or a repurposed 1TB drive) and then use mirrordir or rsync to copy everything over to it, and re-run grub-install afterward to make the "new" filesystem bootable.

But like I said, I'm not a sysadmin, other than for my own stuff.

Cheers