When to replace MC Cartridge

Hi all,

What are the reasons to replace a moving coil cartridge and how does one observe them mechanically?

I've seen the estimates for number of hours and how the acoustic properties muddy. In particular I'm looking for what I should see under a very good microscope, and what happens to various frequency ranges as a function of mechanical changes. Should I see burs, asymmetry, flattening of the tip?

I'm using a SUMIKO blue point special with the defacto elliptical stylus.
Look at michael fremers articles on the subject on Analog Planet .com
One should replace an MC cart if one does not like how it
sounds or if the stylus is broken or if one wants something
new and can afford the price.
@Tbromgard: thanks for the tip, ya lazy non-linker, heh. JK. Here's the link:

Great article with lots of concrete visual examples.