At this time, Apple does not perform PowerBook 2400 hard drive upgrades--either at Apple's Central Product Repair Center or at Apple Authorized Service Providers.
If you would like to replace the hard drive in your PowerBook 2400 computer, you may be able to have it replaced by contacting third-party vendors specializing in storage devices.
Note, however, that Apple does not authorize or support PowerBook 2400 hard drive upgrades. You may void your warranty by having the hard drive replaced, should damage to the logic board or other components occur as a result of the installation of the unsupported device.