After letting curiosity get the best of me, I decided to run a private Minecraft server to test this question, and I've found that you can indeed simply edit the ops.json file and change a user's op permissions level.

 For those that are curious as to how that change will take effect; understand that a .json file is pretty much a running script that never stops until the "host" being the server stops, and with knowing that, an op will not be affected until they re-enter the server after the ops.json file is updated.