How to Verify Java Version Compatibility

I'm trying to re-compile the web app I created in Java 1.4 and run it in Java 8.

If the Java version goes up, there will be various compatibility issues, so please let me know how to check it.

For example, in Java 1.5, enum is a reserved word, so if the source contains the variable enum, Java 1.4 will compile, but Java 1.8 will fail.
As for compilation errors, I can pick them up if I compile them, but how should I detect them if there is a difference in behavior without compilation errors?

Java 1.4 → 8 so
1.4→5
5→6
6→7
7→8 (http://www.oracle.com/technetwork/jp/java/javase/overview/8-compatibility-guide-2156366-ja.html#A999198)

I'm wondering if I should check the compatibility documents for each
All applicable documents could not be found.

I wish there was a migration guide to Java 1.4→8...
Please let me know if you know.