Originally Posted by shore9
How is it a freebie, asides from breaking the rules, which would also be the fault of the LH agent if he was to get the upgrade
The OP wrote that he was "Buying AC B-fare, planning to use SWU
with 4-day window", therefore it would really be the fault of AC and not LH

.
Strictly speaking, an AC upgrade certificate itself is as useful on a LH-operated flight as an AC in-flight duty-free shopping coupon.
Originally Posted by shore9
but he would still have to hand in an upgrade certificate.
To whom exactly would he hand it in? The AC agent while they are talking over the phone? Or to the LH agent at the airport (who will have no reason to even request it and also have absolutely no use for it)?