Hello @KB30 ,

You can try below script in you onChange client script.

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
	if (isLoading || newValue === '' || g_form.isNewRecord())
		return;
	else if (newValue == oldValue)
		g_form.setMandatory("work_notes", false);
	else if(oldValue ==3 && newValue==2 )
		g_form.setMandatory("work_notes", true);
}

Please mark this helpful and accepted if it worked for you.

Thanks

kps sumanth