Try doing below.
<apex:selectCheckboxes value="{!users}" layout="pageDirection">
<apex:selectOptions value="{!userOptions}"/>
<apex:actionSupport event="onselect" action="{!checkSelectList}" reRender="Panel1"/>
</apex:selectCheckboxes>
Please let us know if this has helped you out.
Thanks!
Edit
Check the below snippet
<apex:pageBlockSectionItem id="rectypesec" rendered="{!IF( Case.ContactId == NULL, false , IF(isUpdate,false,true))}">
<apex:outputLabel value="Case Record Type"/>
<apex:actionRegion >
<apex:selectList value="{!selectedRecType}" id="rectypeid" size="1">
<apex:selectOptions value="{!recTypeOption}"/>
<apex:actionSupport event="onchange" action="{!GetTypeValues}" reRender="mainPage" status="loadingStatus2"/>
</apex:selectList>
<apex:actionStatus id="loadingStatus2">
<apex:facet name="start">
<apex:image url="/img/loading.gif" style="width: 20px; height: 20px;" />
</apex:facet>
</apex:actionStatus>
</apex:actionRegion>
</apex:pageBlockSectionItem>
<apex:pageBlockSectionItem id="rectypesec2" rendered="{!IF(isUpdate,true,false)}">
<apex:outputLabel value="Case Record Type"/>
<apex:outputField value="{!caseObj.RecordTypeId}"/>
</apex:pageBlockSectionItem>