When you know which career you want to change, you right-click THAT career's OBJD file, then select "Toggle Group Filter" (or you just select the appropriate OBJD and press CTRL-T). After that, make sure that in the Resource Tree, you've selected the top line (AllRes), so you'll see all the resources belonging to this career. Select all of them (or only the ones you need to change, if you know which ones those are), and use the "Extract..." option. SimPE will ask you where to save stuff.
Then leave SimPE, go to where you saved stuff, right-click the package.xml, and select "Open with SimPE". Now you can edit stuff to your heart's content without the risk of breaking the original files.
Thanks for the help though!
PS: How do I mark a post as "best answer?" I don't see that option anywhere.