There are plenty of tutorials on how to exract and blah blah blah the DunDefHereos.dun save but I have yet to see anyone specifically talk about renaming items to make them something else.
It's really quite simple for Weapons where it'll be like so and so weapon name with a number after it in the ASCII field of the save. Where all you have to do is change the number in the ASCII area for the specific item. You can fairly quickly put together a list to figure out the number for each weapon type for the various heroes.
For example, "CrossbowStyle5" is the name for the Blasticus Assault Rifle weapon that the Huntress uses. Using this renaming method you could convert a bunch of crappy Huntress weapons into a whole bunch of Basticus's.
What's really cool is converting the "Serpent" pet, which is the Chinese Dragon, into a "Chicken", "Giraffe", "Hamster" or "Griffon" and these are all interchangeable. It's easiest to use the "Serpent" as the base pet as it is so widely available in the Tavern's store and you can also get them by reaching wave 15 of Magus Quarters on Survival/Pure Strategy. The reason this works is because the names of these specific pets are all the same length. When you rename them you aren't adding or removing any bytes which would otherwise corrupt the gamesave.
Special thanks to fairchild for their D/C tool as I would never have been able to fix the checksum when I was using offzip/packzip. Thanks.