Yes, replacing this drive would be easy, and would only mean dissconnecting the connection from the floppy drive from the motherboard, taking the drive out, and then putting the hard drive in the space instead, and connecting this as slave (usually if you have another drive) in an IDE slot (There are 2, though 'IDE 2' is usually used for CDRom/DVD drives). So yes you can!
They are small enough to fit into where the floppy drive was if you are limited in space (3.5),
which I think you must be to take out such a neccesary component these days.... I'm not joking actually hehe, I think they can still be useful!
Though people get by with USB pen drives and personal USB hard drives. If you have a musical keyboard that will only take floppy though, you have a problem.
Though if there is room in your case, I'd leave the floppy, and connect the new hard drive as a slave instead as they'd be no need dissconecting it, as you can't use the connection the Floppy drive went in anyway (Thoguh I think you might know this)
Hope this helps