<apex:page id="thePage" standardcontroller="Unit__c" extensions="Unit_Ext" ....

<apex:outputPanel id="fields" ....
    <apex:outputfield value="{!Unit__c.Tenant__r.AccountId}" ....

<apex:selectList multiselect="false" size="1" value="{!Unit__c.Tenant__c}">
    <apex:actionSupport event="onchange" rerender="fields"/>
    <apex:selectOptions value="{!Tenants}"/>
</apex:selectList>