You can use ren (as in rename):
ren *.XXX *.YYY
And of course, switch XXX and YYY for the appropriate extensions. It will change from XXX to YYY. If you want to change all extensions, just use the wildcard again:
ren . *.YYY
One way to make this work recursively is with the FOR command. It can be used with the /R option to recursively apply a command to matching files. For example:
for /R %x in (*.txt) do ren “%x” *.renamed
will change all .txt extensions to .renamed recursively, starting in the current directory. %x is the variable that holds the matched file names.
And, since you have thousands of files, make sure to wait until the cursor starts blinking again indicating that it’s done working.