My wife and I try to do most of the work in the house myself, so we are a bit handy, and was trying to replace the hardware in the bathtub, however, I'm not sure if I'm overlooking something or I should contract a plumber to do this. I appreciate the time you take to read this and suggestions.
Here's the situation: We moved in to a 1994 house that has one of the universal bathtub (Moen) hardware, and after pulling it off I have the following:

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We liked the Kohler "Mistos" set, and purchased it at either Home Depot or Lowe's.
From the Kohler's site (just in the previous line), you can get to the PDF of the instructions on how to install it, but it seems like I need to replace the whole thing that comes from the wall (the cross that is shown in the picture)? Is that correct? Is there any way I can just use the pipes already installed and not have to break wall, etc? I hope I'm just missing something....
I noticed Terry's post on Installing a Kohler Rite-Temp bath & shower valve, K-304 but the instructions are there if you are installing for a brand new house.

Let me know your recommendations. Is it something easy to do? Should I get a Pro to do this instead? or should I just go back to a universal one?

Thank you