Coins that changed their PoW algorithm?

[tmbp]

Have there been many altcoins that changed their PoW algorithm after a certain block? Did any name the cause?

I am looking for links so I could analyze the source and perhaps get an explanation as to why it was done.

[BitsBitsBits]

Many coins use POW in the beginning than switch to POS. You can look up Silkcoin, they are doing it and are currently in the second POW phase.

[tmbp]

Many coins use POW in the beginning than switch to POS. You can look up Silkcoin, they are doing it and are currently in the second POW phase.

I was more curious about coins switching their PoW algorithm after a certain block

[DigiByte]

Many coins use POW in the beginning than switch to POS. You can look up Silkcoin, they are doing it and are currently in the second POW phase.

We are in the process of switching to five algos as we speak. Spaincoin also made a change. There have been a few others.

I was more curious about coins switching their PoW algorithm after a certain block