In the 'onCollapse' event, get the expanding panel and override the 'tools' property. Please refer the code below:
<div class="easyui-layout" style="width:700px;height:350px;">
<div title="North" style="height:100px" data-options="
region:'north',
split:true,
onCollapse:onCollapse
"></div>
<div data-options="region:'center',title:'Main Title',iconCls:'icon-ok'">
</div>
</div>
<script type="text/javascript">
function onCollapse(){
var opts = $(this).panel('options');
var layout = $(this).closest('.layout');
var region = opts.region;
var p = layout.layout('panel', 'expand'+region.substr(0,1).toUpperCase()+region.substr(1));
var tools = p.panel('options').tools;
p.panel({
tools: [{
iconCls:'icon-add'
}].concat(tools.pop())
})
}
</script>