the longer answer is that you could make use of multisession writing to accomplish much the same end result as long as the disk is not finalized after the initial write. Multisession disks do not hold as much data because each session requires considerable overhead.

In whatever DVD burning software you are using look in the help file for multi session disks and the process will be explained.

If it is a file you change often why not use flash media for the file's storage?