|i only just realised you ment diffent file names, if you use the find command then use rename, REN *.fileextention to *.desiredextension|
if you wish to rename them individually a differnt filename and not extension i am not sure, sorry but i didnt quite understand your post so iposted several possible sources.
use the find command with whatever perimeter you require, then in the line below use the ren command
REN "%random%.doc %random%.txt
Searches for a text string in a file or files.
FIND [/V] [/C] [/N] [/I] [/OFF[LINE]] "string"
/V Displays all lines NOT containing the specified
/C Displays only the count of lines containing the
/N Displays line numbers with the displayed lines.
/I Ignores the case of characters when searching for
/OFF[LINE] Do not skip files with offline attribute set.
"string" Specifies the text string to find.
Specifies a file or files to search.
If a path is not specified, FIND searches the text typed at the
or piped from another command.