Opening chokes
 

Is that a job "any ole gunsmith can do," or should I be looking for someone who specializes in it?

I have an over/under 20 gauge that appears to be modified/full (based on imprecise measuring) and would like it changed to improved cylinder/modified for rabbits and upland birds.

pending the gun, another option is send the gun to a choke pro and have them add screw in chokes , then you can swap as you want!
Briley (of briley chokes https://www.brileygunsmithing.com/c-...tallation.aspx ) has done a few guns for me over the yrs, been very happy with them ever since!

That's an option, too. I noticed the Briley name coming up regarding that and reviews were very favorable like yours.

What I can't see is me changing choke tubes very much, so I don't know if it would be worth it.

I would gather the costs to open the choke up as to threading the barrel wouldn;t be a lot different, But having the ability to change chokes is a benefit you won't get by just opening up chokes!
I am sure Briley can do either for you, why not email them and ask for a price quote on the work for each and see!

if you have any good skeet/trap ranges near you, you can also try contacting them as odds are they have either a smith on hand, or know of many close to you that might do work for less $$ than Briley might
as most smiths that are well know on trap/skeet ranges do a lot more shotgun work than the average smith does(IIMO< most basic smiths do more on rifles and handgun work as there seems to be more interest in them over shotguns)

so by going to either a well known shotgun smith like bBiley or one recommended from a shotgun shooting range will IMO get you the better results you will want !
either way, getting a price quote should be free to get!

before you contact them, I would suggest measuring the thickness of the barrels at bore and 4-5 inches down barrel,a s they will need this info to know if they have enough material to work with and be safe when done!

there is also the do it yourself way if your willing to do so, not sure if you can rent tools needed or not, so costs have to be figured in for this if your a handy do it yourself type!