The newer OpenGL classes (GL30, GL31, etc) still rely on functions and enums from the older ones. The screen is still cleared with GL11.glClear(), VBOs are still created with GL15.glGenBuffers() and shaders programs are still created with GL20.glCreateProgram(). The difference is that a number of functions in those classes have been deprecated and can no longer be used (unless you're using the compatibility mode, which kind of defeats the purpose of it). For example, here's a line from when using sampler objects which store how a texture is sampled (filtering, wrap modes, etc) separately instead of inside the texture:
GL33.glSamplerParameteri(id, GL11.GL_TEXTURE_MIN_FILTER, GL11.GL_LINEAR);
As you can see, the old GL11 enums are still used although the function glSamplerParameteri() is in GL33.
I'd like to point out that switching to OGL3 does not only mean that you'll have to use VBOs; the whole fixed functionality pipeline has been removed. For starters, it's impossible to draw anything at all without a shader.