French Mac site HardMac has posted the step-by-step process of changing a Time Capsule's hard drive. Specifically, they swapped the original 500 GB Seagate SATA drive for a Western Digital 1 TB Green Power drive.
The actual removal of the existing drive won't be too tricky for people used to tinkering with hardware. In fact, the whole thing was simple. The Time Capsule immediately recognized the unformatted drive, formatted it and made it available to Time Machine.
They chose the Green Power drive because they're designed to use a varied rotation speed, based on demand so, it's much quieter than the original Seagate (according to HardMac).