I bought a set of new knives in the middle of hay season to replace my worn knives on my 336. I had hoped to get the knives on before my second cutting this year but when I went to get into the bale chamber to take the old knives out I realized that it was going to be more than a quick job so it got put off.
In the mean time, I had crawled into the bale chamber with a handheld blade sharpener (kind of like what you see for kitchen knives) and that got plunger knife through the second cutting without too much hassle. I heard that sharpening the old knives with a grinder can heat the metal too much, permanently altering the cutting edge so that it will not hold an edge for very long. I didn't trust myself to figure that all out (and couldn't find an old-timer near me who knew what they were doing) so I bought a new set of after-market knives from a guy who makes new jd 336 parts from american steel out of new york. Either way, new or sharpening, you really need to take the blades off which from my perspective involves some time and effort. Hope this helps a little.