Originally Posted by HillRider

In the past and when freewheels were universal what you propose was easy. Every bike shop had "cog boards" with every cog available from Sun Tour or Shimano or Campy or all three. You could mix and match to replace worn cogs or alter the gearing to meet your needs. However, the individual cogs were fairly expensive and replacing more than one or two approached the cost of buying a new complete freewheel.

Modern cassettes generally don't make this possible. First, many cassettes have groups of two or three cogs mounted permanently on a "spider" so they have to be used (or replaced) as a unit. Second, modern cogs have shifting ramps and shaped teeth that have to be aligned properly relative to each other to shift well so you can't just buy a cog and add it to the mix and keep the performance. So, individual replacement cogs are no longer available except by buying more than one cassette and doing a mix-and-match where they fit.