Save your money, going to a 9MM when you have .223 is not a good move. I wouldn't risk getting in trouble for something I was not authorized to have. Not to mention your chain of command would probably make you turn it in as soon as they know you have it.
If I did anything, I would get a couple of extra .223 mags. If I'm carrying extra weight, I want it to be ammo.
As far as large belt knives ~ I had one in Afghanistan and wound up shipping it home because it wasn't practical. I carried an Emerson Super CQC 7 and a Swiss Army Knife (Victorinox Trekker) the whole time. Do over, I would have just taken the SAK.
The less you have to keep up with and move around ~ the better.