Can I update an early Airport Time Capsule?
It seems to be possible: Here is an ifixit.com article.

In the comments there the following successful upgrades are mentioned:


WD Red 3TB
WD20EARS 2TB
Seagate ST4000VN000 4TB


At youtube you'll find several movies demonstrating how to replace the HDD (e.g. Apple Time Capsule Hard Drive Upgrade)

Check the power consumption specs of your new HDD as well as the comments about reliability issues of the power supply unit of the Gen 1 Time Capsules all over the net.