Hi,
I think you can do that. You have to restart the VC service afterwards to let the changes be active.
I do, however, not recommend this. It is easy to make a mistake in the xml file and your job file is currupt. I don't know how Visualcron reacts if that happens.
If you are handy, you can do this with the API I think. You can itterate through all the jobs/tasks and do some changes there. That is a more robust solution.
If you still want to modify the job.xml try the folowing:
Make a backup of your settings, using the standard backup job. It creates a zip file and that contains the job.xml. Make a copy of this zip and alter the job.xml in that copied zip file.
After that you can import settings from that copied zip file and only 'update' the job section from it. If that fails, you still have the original backup file which contains the good job.xml
Good luck!
Uses Visualcron since 2006.