I welcome our fungal overlords.
It is awful to see in tobacco, but mold is a fascinating thing. I read a theory of why dinosaurs did not "re-inherit" the earth and it was pretty much down to just a few degrees in body temperature difference. The idea was that they (reptiles) generally regulated their temperature with the sun, whereas we (mammals) regulate temperature with our brain and can more easily induce fever to kill off fungal invaders.
The theory was that reptiles were much better positioned to take hold again, but fungi kept them in check for the next go round when light was not so favourable for their conditions after some cataclysmic event. Interesting idea.
And... in the long view, apparently some fungi are making progress to erase our evolutionary gain on them. They're patient. And numerous. Something to think about if you're ever considering smoking a blend of mold because it is just too rare or precious not to. Maybe you're the next guy who is "new to science". At least you'll be quasi-famous in a journal article somewhere.
So, I don't really have anything useful to add in regards to tobacco, except that in the rare cases of mold, my returns were prompt.
That being said, I would be very reluctant to transfer anything to jars. If you take a sealed container that does not have mold in it (which is probably the majority of your purchases) and unseal it. It seems more likely that you will introduce mold to your tobacco than if you just leave it be in the packaging. Unless you have access to a laboratory "clean" room where you can do such a thing with relative confidence that there will not be contamination.
It will likely take more years than I spend reading pipes forums, but I suspect there will be a slight uptick in mold from "cellars" of mason jars. If I were a betting man, and I'm not, I would bet on that. You just may not notice it until it's rehydrated to a certain level and the fun begins.
And, if you have time, a fun documentary called "Fantastic Fungi" by Paul Stamets is beautiful and interesting. It reminded me that I need to hunt down some hippies soon to get some more of that special fungi. If the whole world collectively ate a handful of those on Tuesday, my other bet is that we'd all get along a hell of a lot more. I've yet to meet a mean person on the other side of that wall. But that's another post for another place.
Your best bet, I think, is to dry the tobacco before "cellaring" to reduce chances of mold.