I have an old Samsung Galaxy Note 8.0 (GT-N5120) tab. It comes with an S Pen. I wonder if this pen tip could change or not. I did not remember for this is a gift for my family member a long time ago.
Yes, they are called "nibs". Note that the Samsung Note series uses a special type of styli(?) — the technology which I believe is proprietary to them, or at least to Wacom whom they have acquired — called electromagnetic induction-type stylus.
Therefore you can't just get any aftermarket stylus tips (though I have seen some aftermarket styli that I've confirmed works on the Samsung Notes. Also note that the Samsung Note series incorporates two separate touch digitizers beneath the screen: one for touch inputs, and the other for stylus inputs.
It is possible that you can use the wrong type of stylus and still have the device register your inputs, but note that if it's registering these inputs as normal "touch" inputs, you won't have any of the stylus-specific functions (such as the stylus cursor, depth/pressure input effects, etc).