There is always a workaround here at TGC
You can grab yourself a free copy of a tool called NotePad++. You can then highlight all your FPE files in one go and drag them into this tool. It will load all the text files into it under tabs. You can then perform a Replace All function across all files and look for something like:
textured = barrell_acid_D.dds
and have the replace tool change it to something like:
textured = newrock_D.dds
You then do Save All, and next time you load those entities into GameGuru, it will use the new newrock_D.dds texture if found alongside the entity file. Don't worry about BIN files, we deleted them a while ago so the FPE change is reflected the moment you restart GameGuru and re-load your level.
PC SPECS: Windows 8.1 Pro 64-bit, Intel Core i7-5930K (PASSMARK:13645), NVIDIA Geforce GTX 980 GPU (PASSMARK:9762) , 32GB RAM