You should be ok to use the partially used cartridges in the 8400. The printer uses about 20% of the ink to charge the lines and initialize the print head. Since the printer comes with 330ml upfront (which is plenty to start it up and print for a few months), using a cartridge that is at least half full will be fine (for safety).
The printer and software (low warning notification) will tell you when it's getting around 10% to be prepared for replacing.