Changes required to migrate from Oracle JDK to OpenJDK
Now my question is will it be a good idea to migrate to OpenJDK
For Java 11 you might want to, though by Java 11 they will be almost identical.
So do I need to replace all Oracle JDK jars with OpenJDK jars
I would install a version of OpenJDK and use the JARs which came with it. I wouldn't mix and match them.
JDK 1.8
I am not sure gain anything by migrating Java 8. Oracle Java 8 is still supported at least until Jan 2019. After that, you might not get any update, but you might not with OpenJDK either.
If your concern is getting support for Java 8, I suggest contacting a company which will give commercial support Java 8 such as Azul for what seemed like a reasonable price. https://www.azul.com/downloads/zulu/zulu-windows/