'Send Change to Workflow' action is not available in Change Management Process Event rulesets. It is available for Change Management Data Event rulesets and functionally similar 'Send Incident/Problem to Workflow' actions are available for respective Process Event rulesets.
'Send Change to Workflow' action was available in Servicedesk 7.5 MP1, making this a regression.
Pointfix that includes a fix for this issue (along with new automation ruleset at CAB voting) is available for download in TECH212326.
Steps to apply the fix:
ServiceDesk 7.5 SP1