Sooo... looking at the Carrion Lancer and Rune Golem expansions, there are at least 3 siege upgrade cards in each box...
If we assume that two in each box are the same as the ones that come in the Core (Frontline Rune Golem, Support Lancer ... one for each mini), then there's at least one new Siege upgrade in each box.
That being the case, I wonder what they may represent? Would these be upgrades giving the alternative ability of the units? For example, rather than spitting Blight, the alternate Support Lancer may trade that for the ability to apply conditional Mortal Strikes, per the solo unit, and the alternate Front Line Rune Golem might be handing out Stun tokens instead of adding Brutal to the unit.
N.B. I know the article states 8 new upgrade cards, but I've got a sneaking suspicion that that's misleading and we will be seeing the same siege upgrade cards from the core, plus the extra(s)