... are available but usually have much bigger capacities now.. not that it will benefit you because the Panasonic software will only recognise the original size fitted.
You would need a torx screwdriver to change the drive. I'm not sure of the size , but I'd guess at T9 or T10.
As to what happens when you fit it depends on the vintage of the machine. Older units have the operating system stored elsewhere and so disc replacement is possible.
Newer units store the OS on the hard disc and require an install disc.
Caution is required when experimenting because reattaching your existing hard disc can be detected under certain circumstances and the drive will be automatically formatted, erasing all your stored material.
What is your reason for wanting to change the drive?
The machine basically stopped working and when I took it to Panasonic, they informed me that the hard drive had given up the ghost. It's around three years old. A replacement could have cost as much as £150, or they could have fitted a cheaper type of HD for around £30. I took the option of buying a newer machine. The EH60 is just sitting here and I thought about having a bash at repairing it myself, but this is maybe to onerous a task for someone who let's face it, is not that competent in the field of fixing such things!