Can a 29-2 accept a 29-3 barrel and vice versa? I don't like the length and balance of the barrel and want to replace it with a shorter one to make it feel more comfortable to me.

The un-pinned barrels can be installed on a pinned-barrel frame. You need to file a groove on the barrel shank to clear the pin.

When changing barrels, you may have to cut the barrel shoulder and the end of the ejector rod shroud back a full turn, then shorten the end of the barrel to get the proper barrel/cylinder gap. All S&W barrel threads are 36 TPI. I just replaced a 6 inch barrel on my 686 with a 4 inch one, and I had to set up the barrel in my lathe to turn the shoulder/shroud back enough to get good lockup when the barrel is tightened. I then had to cut the end of the barrel to get my B/C gap where I wanted it, and ended up with a .003" gap. I then re-cut the forcing cone to get the correct diameter. I have a good set of nylon barrel blocks for the S&W's, and a good frame wrench too. I also have Brownell's tools for cutting the forcing cones and the gauges to measure them.

I have changed several barrels on S&W's over the years and have had no problems doing so.

Does it only work this way? Or can pinned barrels be replaced in un pinned frames? Just asking to satisfy my own knowledge.

The older pinned barrels will fit on an unpinned too.

All of the usual comments about regulating B-C gap and timing the front sight so it stands vertical apply.