Replacing the IC (atmega328) if lifespan has been arrive
You only need to change the IC, not the entire board. The microcontroller IS the IC.
Also, you will not see any degradation, it will just 'suddenly' fail, or starting to fail sometimes, without any warning or counter that counts up/down to 10,000.
Also keep in mind that this number is the MINIMUM guaranteed value, most ICs will go much further.
The best way to prevent getting into problems related with this issue is:

When you have a finished project and it works, just keep that IC for that project.
In case you fear getting into this problem during development, only change it if you are sure you have problems (i.e. you cannot upload your sketch anymore).
Think before uploading 100 times per day a sketch for debugging purposes (add Serial.print lines, or even better, use a PC to debug all non Arduino related code.