You can change filenames in batch by using the freeware tool "lupasrename".
I am sure it works for folder and filenames. You can select a group of files or folders and
> add or delete txt
> replace txt >>> this is the one you need !!!
> Crop left or right with N characters
> autonumber

I am not sure whether your files are all in the same folder or not. In case not, you probably will have to group them in one folder, then do the filename transformation, and then put them back in their respective original folders again.

Hope this helps