How to change the owner of a scheduled report?

Hello,

We have multiple scheduled reports which have suddenly stopped working.  i.e. Stopped sending out the emails with reports.  The underlying reports work fine - don't time out or anything - they run as normal. 
If we recreate the scheduled reports as a different user they seem to work.  Though, strangely, the owner of the original scheduled report had no change in permissions.

Two questions: (1) Any idea why a scheduled report would suddenly stop working?  There end date is way in the future - 2038 - so hasn't expired.  (2) Is there a way to change the owner of an existing scheduled report?

Thanks,

Dan