Hello guys,
I'm a noob with the GECK and I want to change a creature's model in game to some other model from the game.
Can someone please guide me on how to do it the simplest way? I figured it shouldn't be too complicated to simply change a model.

Also I've tried searching the forums and couldn't find anything
Thanks.

Its not too hard to do, but in many cases it won't work right either. Each creature has its own animations, so changing the model can bork the animations.

To do it, you have to extract the model NIFs (skeleton.nif & .nif) from the BSA so you can slect it in the chooser. A utility like FOMM has a BSA unpacker. Once you have the models extracted to the proper folder, you can click on the model button on the creature's model list tab and navigate to the folder you extracted the nifs to and then select the skeleton.nif, then pick which model version of the creature you want. Like all the ghouls use the same skeleton.nif in \Data\meshes\creatures\Ghoul, but each variant has its own model nif.

If you do assign a creature a new nif, won't you just be making a copy of a creature that already exsits anyway. And if you are trying to take a fire ant's nif and apply it to a ghoul, the animations will get borked anyway.

Thanks for the reply,
So if I do want to change for example Fire ant's model to Ghoul and have the animations not get borked, do I need to also extract each of the .kf files (I presume they're the animations files)?

I think that would be nearly impossible, you'd have to rig the ghoul model to the ants skeleton which I assume would take alot of experience.
Someone correct me if Im wrong.

BTW, I'm trying to do what this guy has done here but to different models: http://www.fallout3nexus.com/downloads/file.php?id=2556

That's pretty much true. The only person I know of on these forums that can do that is Saidenstorm. He helped me add firebreathing to the brahmin. If it wern't for him and ThrottleKitty, I never would have been able to do it myslef. And that was just adding somehting, not making full-scale changes. But I am no expert, so wait a day for some of the modelling pros to add their comments.

If I understand it corerctly, when changing from one creature to another that is similar in its skeleton, the change is more likely to work. I think the roach and the rat have similar bones.

Yea, if you tell us what two things you want to switch, we can probably estimate if it would work easily or not.

I see, then I'm planning to change ants, bloat flies and scorpions to moles.
I'll give it a try. Thanks.

Ants maybe
Bloat Flies I think that be near impossible
Scorpions maybe but the tail would be a problem me thinks

Yea like The Gate Keeper saids. Ants should work, Scorpions should work. Bloat flies, will not work at all without some MASSIVE modification to them.

Hey guys I found a mod on nexus already does what I wanted to do, wish I'd found that earlier: http://www.fallout3nexus.com/downloads/file.php?id=7691
He managed to replace a large number of creatures to ghouls without much trouble.
Thanks everyone for replying.

This basically has sod all to do with changing things ingame and there's no reason whatsoever you need to stick with the same skeleton. You can easily do it in the editor, and it's trivial in TES4Edit- select everything you want changed along with the molerat, right-click, compare selected.
Then find the Molerat's Skeleton and Model entries in the window and right-click, copy to all selected.