If you have no more pens to upgrade, here's a quick-and-dirty fix:
Sell off a family of animals with a short breeding period (beagle or shorthair cat, I think.) You can repurchase, breed new ones, and then upgrade the pen twice. I know it's kind of a drag but if you want to finish the goal it's probably your best bet. Or maybe they will add a new coin animal tomorrow? One can hope