It's an easy job for some, a big job for others. I can't say where you fit on that scale.
Once the pan is off, the valve body is right there. You'll want a diagram (try www.atsg.us for a shop manual) before you start. Once the valve body is off you'll have to locate the 2-3 shift valve (the diagram will show it) and take it out.
The two codes each say that third gear isn't working.
Since reverse works fine the direct clutch is probably fine. That leaves a sticking 2-3 shift valve as the most likely culprit. That's located in the valve body in the pan. I've seen them work at one temperature and stick at another. I think that's what you have.