Excellent questions, all!
First, a couple of definitions. I will use the terms TDA (Traditional Double Action) to mean DA/SA (Double Action/Single Action) like on the 5906 and DAO (Double Action Only) to mean, what is in reality, the "partially cocked" (is that like "kinda pregnant"?) hammer with a long trigger pull, single action like on the 5946.
The fundamental difference in operation between the two is brought about by the substitution of only two parts: the trigger and the hammer. Specifically, the difference is the location of the hooks on the trigger and the notches on the hammer.
(BTW, on the older DAO models with silver triggers and hammers, the hammer and sear were produced and sold as a set. On the later, blued trigger models, both TDA and DAO models used the same sear.)
Take a TDA 5906 frame and swap the trigger and hammer with those from a DAO 5946 and the 5906 frame will operate like the 5946.
The same holds true in reverse, i.e. swap the 5906 trigger and hammer on to 5946 frame and voila, DA/SA.
WARNING!! As Sevens has accurately observed in his post, "it wouldn't have a decock or thumb safety" and could create operational safety issues. Indeed, the early, silver trigger/hammer DAO models were produced with a frame that was not machined to accommodate the sear release lever necessary for safely decocking the TDA pistols. In the interest of simplifying production, S&W eventually commonized that aspect of machining and made them all with the wider TDA cut and installed a "cropped" sear release lever to act as a spacer (the third part in the "four part" substitution plan).
On to the slides. In the DAO models, S&W installed a "firing pin retainer" (essentially a block of steel machined to emulate the manual safety in the "ready to fire" position) in place of the manual safety (the "fourth part" in the conversion). On all standard DAO model pistols as well as pre-rail TSW models, S&W machined the slides differently so that the safety parts could not interchange. Once again, in the interest of simplifying production, on the latest railed TSW models, the factory elected to machine both TDA and DAO slides the same and instead produced a "firing pin retainer" that fit in the manual safety cut out.
Not included in the above is the physical difference in the DAO frame and slide vs. the TDA in that the DAO frame and slide are longer at the rear to shroud the partially cocked hammer but that makes no difference in the operation.
If there's anything I did not make clear (like all of it) please ask for clarification and I will try to explain.
I hope this helps!
John
) is really great news.
I have a 5906TSW produced for CSPD which has a "Decock-only" lever (I'm still not sure what S&W terminology is for this) and I'm toying with the idea of converting it to a traditional "Manual Safety" lever if that's feasible. I'd be curious if this is something that can be done in the slide or if it needs more parts or is even feasible at all? 
