A lot of JCK source files has predefined class file version 49 or below:
jcod: class Test {
  0xCAFEBABE;
  0; // minor version
  49; // version
}
jasm:
public super class Test03 version 49:0{}

The new option introduced for the JCK build process is -fixcv. The purpose of this option is to convert old classes in the JCK to new ones by overriding their class file version with the specified <major.minor> version. This allows the JCK to be updated to support newer Java versions without having to recompile the entire codebase.

For example, if you specify -fixcv 49.3-52.0, the old classes with version 49.3 or below will be modified to have version 52.0 after the JCK build process.