It appears to me that the frames on P239s are all the same other than the 40/357 mag wells being slightly larger.

So, my question is can the DAK trigger mechanism on one be exchanged for a SA/DA one?

What I want to do is buy another 9mm P239 with a SA/DA trigger and trade it with a DAK trigger on a P239 357sig.

And how hard and difficult would it be for me to do this? (I once took the trigger group apart on my Sig P230 and it took 8 hours for me to figure out how to get it back together)

I am not sure if the DAK trigger could swap from an regular 239 frame to DAK. I know the P226's frame must be made for DAK trigger style.
The trigger work is not really hard if you are an handy person. I love Sig hand guns because the simple internal of them beside the beautiful design. There are many Youtube videos for trigger work on P239.
Hope it help

Yes. They can be converted. Here is the parts list:

Parts to convert from DAK to DA/SA
Decocking Lever
Decocking Lever Spring
Decocking Lever Bearing
Hammer
Hammer Strut Pin (except P239 which is integral) *Note* Recently, #34280325 and #34280325N have been coming with the Hammer Strut Pin already installed
Hammer Reset Spring
Hammer Strut
Mainspring
Safety Lever
Sear
Sear Spring
Trigger Bar

Sig had one time would perform the conversion for $200. Parts included.

Both of the DAK 357s I have have the grips with a cut out for the decock lever....

I guess there will be one way to find out and that is my going ahead and picking up a SA/DA P239 in 9mm... and just seeing... the good thing about this is while doing the change out I will still have a gun with either of the trigger groups I can refer back to.

To explain my thinking... I'm not totally sold on the DAK trigger having been shooting DA/SA for years So, I have upgraded to 357sig for carry and would like to have both choices in a P239 which is at least for now my favorite carry pistol... but which ever I carry I would prefer to shoot 9mm for any length of time while at the range. I'm both cheap and a whimp.