I change my copper tubing to flex tubing. Which piercing kit do I use now?

If you are using a saddle valve then all saddle valves work on copper tubing and plastic flexible tubing. The only difference is that you want to use a brass insert and a plastic ferrule on the plastic tubing going to the saddle valve (and where the tubing connects to the refrigerator)... see image below.

With that being said, I don't like saddle valves (illegal to install in most states)... seal can dry and leak over time. You would be better to install a proper shutoff valve here, but that is up to you. I am glad to discuss more if you want, OK?

Mark