change file extension

[danga]

can someone tell me
how to edit a command to change
only the extension
of all selected file

I tried

{
RNFR %f
RNTO %f.bak
}

but this add the new extesion to the previous one

what I'd like is something that does

*.one ->*.two

thanks a lot

Danga

[bigstar]

I'm afraid that's not possible.