Yes It's a big change that should give more value to cards that add more slots to your board. I am keeping a close eye on this and would love some feedback to see if its working.
Viewing post in Upgrade rework, Better tooltips & More | Patch Notes comments
I definitely find my self holding onto the explorer for longer, making it free with the upgrade and buffing it with other cards.
Temple also becomes a good pick: I often steer well clear of structures because taking up a slot with no attack action is such a penalty.
Hard to imagine myself wanting to take piranha, unless I'm going for an "on death" strategy.
Sun stickers become more of an appealing pick for sure.