Hello,
in older versions of Alfresco (Community) it was possible to avoid a restart by using the Workflow Console (part of Explorer UI) to upload newer version of BPMN20 XML files for Activiti. Unfortunately, since Alfresco 5.0.b, the Explorer UI has been removed and with it the Workflow Console. So at the moment - unless you write your own tooling for online upload - restarts can't really be avoided when using the Spring bootstrap approach for workflow definitions.
Regards
Axel