Changing HDs on PS4 does not void warranty. Changing it doesn't break the warranty seal sticker.
They even show you how to do it.
https://support.us.playstation.com/app/answers/detail/a_id/5107/~/upgrade-ps4-hdd
They even show you how to do it.
https://support.us.playstation.com/app/answers/detail/a_id/5107/~/upgrade-ps4-hdd