Unfortunately, simply renaming them with the same name won't replace the textures.
You'll need to open the signtest.p3d in Oxygen and manually replace the texture. That's just a case of doing some simple operations in O2:
1. Press Alt-T to open the texture window. You should see the .paa file with 'Bitrov' on it.
2. Then, Ctrl-Click on the texture in the texture window (this will select all the faces that that texture is applied to) and press 'E' to bring up the face properties dialogue.
3. At the bottom of this dialogue, you should see a text box with a path to the bitrov.paa file. Merely change this to 'signtest.paa' and save signtest.p3d. Replace the files (once PBO'd) in your Addons folder and you're all set.
You can get O2 (if you don't have it) from
BIS's Breathe site.
Also, some good tuts as well as a how-to-set-it-up are available from
Brsseb's website.
editI just noticed that this should probably be in Addons:Editing or something (well, not scripting
)
edit 2Yeah, just noticed 456820 posted while I was writing mine - that's probably the easier method