Display value of the Old foreign key

If I want the display value of a foreign-key object, I can do this easily using [d] notation, e.g. $UID_Department[d]$ from Person.

If I want the old value of the UID, I can do this easily using [o] notation, e.g. $UID_Department[o]$.

But how do I get the old object's display value?  It doesn't seem to accept $UID_Department[od]$, $UID_Department[o][d]$ or $FK(UID_Department[o]).UID_Department[d]$ all I end up with is the raw uid value.

Is there an easy way to do this, or do I need to explicitly open the object reference by the old UID to get it?

Thanks,

Neil.