It seems like it should be easy to go up a numbered drill big size or two in a barrel for more gas after cutting a barrel shorter.  Seems I just build a jig to keep the barrel straight, put an old cleaning rod in the barrel, and set the drill press depth.

Then use plenty of rpms and oil.

Is it really this simple or is this a job for pros?  Will burrs in the barrel be a problem for the garage crowd or will the first magazine clean up the edges?

You're using small, brittle bits that break easily.

If you actually have a fixture holding the barrel tight and true, then drilling larger is just that easy. If the barrel can move around at all, or your fixture isn't dead-on balls-accurate, you risk the drill bit grabbing an edge or flexing, and breaking off.

The only part about it that requires pro-level tools and talent is making sure the fixturing is tight and true, and even amateurs can get lucky free-handing it.

It is as easy as OP describes, but just be sure you really need to open it up.

just my opinion, if your drill bit is nice and sharp (i.e. not dull with high-mileage) and good quality, then, if your feed rate is really slow, the drill swarf (chips) should be pretty small and easy to break off, leaving a nice clean transition at the edge of the gas port hole in the bore.

check the alignment by sticking the drill bit of the existing size in the barrel gas port, and the prospective drill bit in the chuck, bring them down next to each other, and make sure they're parallel to each other from from all angles.

those aluminum barrel vise blocks in an x-y cross vise help.  i wouldn't do this without a drill press.  and be sure to calculate the percentage increase in port cross-sectional area that you're shooting for, as a minimum calculation, when figuring out how much more to go.

I find it easier to use a reamer turned by hand to adjust gas port sizes.  A reamer follows the existing hole and is less fragile than a drill bit.  You're only removing a little metal so there's no nee for power.  Drill bits can do funny things when you're only removing a few thousandths of metal, and if one breaks removing the stub is a PITA.

Ive done several all using a T handle bit wrench (similar to a tap holder only for drill bits) with great success. You can be much more careful doing it by hand and letting the bit do the work and it follows the original hole nicely.

I've done this a couple times.

I'd say run the drill at low rpms vs. higher rpms and don't try to jam it through.  Go slow.

Both time I had no burrs to remove, but I did have a factory barrel with a burr hanging down from the gas port once.  I used a brass cleaning rod (minus a tip) to keep smacking the but till it broke off.  Hit is in one direction a couple times, then the other direction a couple times and then repeat and it will break off.  Clean the barrel and it should be ready to try out if you also got the gas block re-installed.

Get a gunsmith to do it if you can... especially if you don't have a lathe and the proper reamers.

You can't go inside the barrel and deburr it so your hole must be ABSOLUTELY burr free. This is done by using reamers and going very slowly and removing a tiny amount of material per pass. So you will need a lot of reamers in small increments. Don't use a twist drill bit because I've found they're not all that good at making perfectly round holes and would often catch and break.

If you got the tools it's not a hard thing to do, but the tools could cost a lot because you'd need a bunch of reamers.

It's an easy process, not nearly what some make it out to be. I've done it with nothing more than a vise, the barrel, and a cordless drill with the correct sized bit. You're literally only removing a few thousandths of material. The first time I opened up a port was on a 20" rifle gas BCM barrel that was cut to 16". I couldn't believe how easy it was. The bit was through before I knew it and I was done. It was like a hot knife through butter. Going from, I believe, the original port diameter of. 093 to the desired diameter of .100 was so simple, it's not even worth mentioning. Just be sure that you actually need to open the port before doing so. I've seen some carbine gas barrels that were so overgassed to begin with that they ran just fine after being chopped to 10.5".

I agree that it is a simple job. Drilling an existing hole a little bigger is not brain surgery.
Take your time and don't drill into the opposite side of the barrel. Clean off any burrs with a cleaning brush. The first round fired from the barrel will do the rest.
Cutting and recrowning the barrel is fairly simple as well with some basic tools. Threading the barrel is going to need specific tools that most people don't have laying around so having it done by a smith may be a good idea. If you plan to thread more than one barrel in the future buying the tools and doing it yourself may make more sense.