Thanks for all the help. I was aware of condor_now, but I explicitly didn't want to remove a running job (my poor users...), I wanted to run this job next, once a slot would become available. (In practice, that would mean waiting about 2 min.) We recently switched from SGE as a batch system, so I was looking for the equivalent of qalter [jobid]. The job in question was submitted as a debug user which tends not to have many jobs in the queue, so for this condor_userprio is probably the way forward. But for cases where a user has a lot of jobs in the queue, I would first have to re-order these jobs internally using condor_prio and use then condor_userprio to give that user priority.
I guess what I really need is a "condor_inamoment" as a milder version of "condor_now".
Regards,
Daniela