This tutorial will go over how to change the personality of a breedfile, and ergo changing the personality of the pets adopted from it. You should only swap personalities of files you have created yourself. It is not appropriate to open other individual's files and edit them in any fashion.
For this tutorial, I chose two very opposite breeds. I'm going to create a bulldog that acts like a poodle.
To get started, we're going to open both breeds up in LNZ pro. As the bulldog is the final look I'm going for, I'm going to open up his adult LNZ section.
Click the + symbol on the left, beside "LNZ," and choose adult. Copy it, and paste everything into the other breed's adult area. Do the same for the pup/kit area. The breedfile of the personality you want should now have the information of the look you want.
This isn't it, though! If you save your end-game breed now, your game will crash. Open the adult section of your personality breed: in my case, the poodle.
Notice the areas I've starred. These areas are still corresponding with my bulldog breedfile, which is what's making your game crash had you tried to load it sooner. "BD" are bulldog initials and do not belong in the poodle file. The initials for the poodle, as you can see on the left, are PD. Change these areas to reflect the breed you're working on. You will find remnants of your old breed in your pup or kit area as well, and they will need to be modified in the same fashion.
What was once \ptzfiles\dog\bd\bdsnd.txt, and bdpup.lnz in the adult area, are now \ptzfiles\dog\pd\pdsnd.txt, and pdpup.lnz. In the puppy area, you only need to worry about [Default Linez File], and change it just as you did the adult area. Save!