v6 Migration Guide
PixiJS 6 comes with few surface-level breaking changes. This document is not complete.
Typings
If you're using TypeScript, make sure the follow is added to your tsconfig.json:
{
  "compilerOptions": {
    "moduleResolution": "node",
    // Required for importing 3rd-party dependencies like EventEmitter3
    "esModuleInterop": true
  }
}
Mesh Internals
If you ever overrode Mesh._renderDefault to take into account more uniforms like this: v5 Reference
if (shader.program.uniformData.translationMatrix) {
  shader.uniforms.translationMatrix = this.transform.worldTransform.toArray(true);
}
Remove the if, leave the contents, otherwise you might not get correct sync uniform for translationMatrix, or even worse - get null pointer. v6 Reference.
shader.uniforms.translationMatrix = this.transform.worldTransform.toArray(true);