Sure, you just need to go to a certified apple center to do the upgrade, otherwise you could screw up your Apple Care warranty, if it is currently going.
If the warranty has void at this point, you could do it yourself, but it may be risky. Depending on the model and release date of your powerbook, the internal HDD specs may change, so be sure of where U go to ask help with this techy job.
Welcome to the forums Spyrte.

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