I second mazeframe with the tiny files and fret saw. Theyre meant for very finework on guitars and work well here too. The problem is they can be less common and expensive.
Would a drill or reamer not do? Still a little risky and maybe i am misunderstanding the work needed. But it seems a graduated hand reamer may help here.
That is actually a perfect rotary tool use case.
I used my Dremel for doing the same while modding a gameboy case.
The dremel disks are just a bit too big, the Proxxon “circular saws” Name is NO 28 830 work nicely though.
A drill is a big ehhhhhhhhhhhhhhhhh as I’m concerned about taking out more material then is needed, only to find myself in a much more difficult situation then what I need to be. A reamer would be the option I’d go with if I was adamant about not using a drill, and other options just couldn’t pan out.
The fret saw would more then likely be a poor fit here, given the space I’m working with and the fact that I’m not wanting to do any damage (if at all possible) to the exterior. As for the tiny files… Don’t know what I would be looking for to be honest. While I’m sure, if done correctly, it could make for a nicer job. Though at the same time, I’m not really concerned about that being the case, as without opening up the headphones again, I’ll never see it.
Once again it looks like you want to either cut out everything between the standing tabs or make the inside hole bigger to fit the new connection? Am i right?
I think a reamer would do well.
But once again i dont know the mod extremely well or the sizes of things so take my input with appropriate sides of salt.
Edit: it looks like i didnt fully read your response but ill leave this as is for any reference
@Souldriver @MazeFrame well… I found an… unconventional way that I’m not going to disclose. But it seems to have worked.
Ahh i see you gnawed it off. Just check your teeth aftee
Done! Definitely the trickiest as it didn’t really wanna cooperate. Now, to see if it still works single ended (ordering a cable from HAC)
I FUCKING DID IT AND IT WORKS!!! Thank you @ZeosPantera for having the “Left. Right.” stereo at the beginning of your sound demos, makes it really easy to verify that the channels aren’t mixing together and both sides work. I can NOT describe how anxious and scared of how I could have screwed myself over on this one.
Congrats! Nothing more satisfying than finishing a soldering project. Testing it out and its perfect!
Nicely done!
The “nibble away the plastic”-method is 100% valid. 
Particularly when I’m not exactly in the position to replace them should things go that far south
Not sure how i got turned on to this track might be from here. But hope this helps.
My test was always pearl jam - do the evolution. It pans guitar in the begining
