The tips are changeable. My Note 4 came with replacement tips. I would imagine that they should be replaced when they start to flatten on the end of the tip.
Yeh - from a Note 3 they're easy to change. You can be brutal with clamping and pulling one out, then pushing a new one in.
If the pen doesn't feel right, or the alignment is off using the on screen dot setting. I changed mine once in 18 months (early), and there was an improvement.