So, I was thinking about making the jump to Japanese definitions with my vocabulary flashcards, but I was wondering if there was anything quicker than just manually copying and pasting the Japanese definitions over the English ones: I know there is.

I made most of my vocab cards manually (that is, copy and paste from edict), but that's a pretty process: I have some 6000 vocab cards, so doing that manually is out of the question.

In theory, it would be possible to do what you describe with a plug-in for Anki.

In practice, you may run into "lost in translation" sorts of problems. In the end, you would probably need to manually check each card anyway (although, admittedly, this could be done as you're reviewing the cards, and would take less effort than entering data manually).

Yeah, I was thinking about just changing each card as it comes up, which would be easier to spread out over time, but still take a long time.

Personally I think it's a waste of time. Once you know what a word means, you're not going to gain a great deal by revisiting the definitions in a different language. You may get some reading practice and it may be an interesting exercise into how the Japanese definitions differ, but I really don't think it's worth it. Just switch to J->J for new cards (I'm not convinced that is a significant improvement over using your native language either, but that's an argument for another day)