The Job Prioritization defined at a Database Instance level defines how many simultaneous jobs for each priority level can simultaneously execute SQL against the data warehouse. For more information regarding Job Prioritization and Connection Management in Strategy Intelligence Server 9.x, please refer to the following Community Article:
To change this information the 'Job Prioritization' tab settings, under a particular Database Instance, should be modified:

After making changes to these connections the Strategy Intelligence Server 8.x/9.x will not need to be restarted. However, it is possible that there are still active cached connections for the modified database instance. These connections can be checked in the Connection Monitor:

If there are cached instances, then any reports or documents which execute and use the cached connection will still abide by the previous number of allowed priority connections. If no jobs are currently executing, then these cached connections can safely be manually closed by right-clicking them and choosing 'Disconnect':

New connections created will use the updated settings.