How big is the difference between OpenGL 1.x and all subsequent ones?

Question

OpenGL

I am constantly confused in learning OpenGL, I learn this GAPI from very old books in Russian (because the new version is all in English), respectively, the book tells about the old version of OpenGL, where glBegin / glEnd is actively used to which I already very accustomed.
So, will the application lose much performance, or perhaps some other problems will arise if it is written using the NEW version (3.4), but the code is executed in the style of the old OpenGl?
How hard can it be to move from a glBegin/glEnd scheme to a scheme with buffers and glDraw###?