Hi Rytis,
I am looking for a similar functionality as you as we need to be able to run lot of update jobs for dozens of repositories.
Tested in my sandbox to change the repository for dummy job with update-statement and ran the dummy job. The job got the repository reference correctly (all it did was to print the repository name out) and I was able to run it.
I was happy with that but chose not to go further as we have problems with jobs that are executed manually in MMC..
Change the repository 5 times in MMC and the passes in the job lose the repository (same as the problem in this thread: http://scn.sap.com/thread/3695370).
Changing the repository in MMC changes also the job name, but changing the repository with update-statement changes only the repository in job. So maybe there is more required than the update-statement (or maybe not - no idea).
regards, Tero