Changing file names?

still fairly new to the whole “burning dvds” and am slowly making progress. my next question is: (using nero) when adding a second video_ts folder to the project(i.e. another movie-multiple movies on one dvd) what file names can i change so nothing is duplicated but will still work? at the moment when ever i make a change such as changing the .vob file (i.e. vts_01_3.vob to vts_01_4.vob) i get an error saying that this file may no longer be playable. am i changing the wrong part of the file? the same goes for the .IFO and the .BUP names. help!!!