There should be no headspace issues with a new barrel. The 9mm AR platform is straight blowback, so it's a lot easier to change a barrel vs a 5.56mm rifle. There's no gas system to swap over, and the 9mm cartridge headspaces off the mouth of the case, so that won't be an issue as long as the barrel is in-spec. You can use a headspace gauge to check it, if you want to be thorough.

Basically, remove whatever handguard/forend you have installed first. Then remove the barrel nut, install the new barrel, and reinstall the barrel nut. Depending on what type of handguard and barrel nut you're using, you may not even have to worry about aligning a notch for a gas tube. Some rails have to be aligned anyways for them to go on straight, but with some models, it doesn't matter.