regularly copy files to different directory with different file extentions

view full story

http://www.unix.com – I have a NAS and I upload my videos to it from time to time. the video format is 3gp but my media player PBO could only play avi. I want to copy these files to different directory and change the extention from 3gp to avi. (yes I want to keep the original version and create a duplicate version with different extention). Could anyone give me some hint which commands I should start to learn for this work. I want my arch linux to check the original folder daily for any newly added files. if there is any new files, then do the copy and change job. Thanks! (HowTos)