When I retrained after 13 years it was indeed a question of starting again from scratch, but I went through the course at about twice the original speed. (Turned out I'd more-or-less remembered how to drive an aeroplane around the sky but had forgotten all the procedural stuff. YMMV of course.)
What you need is training as needed to pass the test, there's no particular number of hours specified. (Unless it's all changed yet again, of course.)