I would think that all it takes to shift from a believer to a knower is one unquestionable encounter with one of the big furries.... As a believer, they would probably have a number of theories as to what they might be/do, and upon actually observing one or more of the creatures some theories will most likely fall away in favor of those that are supported by what the individual saw.
Through the elimination of theories and hypothesis that dont effectively address the observed phenomena, one is left with various explinations of various aspects that form a framework of interpretation which will at least imply further probable conclusions extrapolated from the event, as well as from the reports of others having had similar sightings with parallel elements involved. So its easy to imagine how a person might become an "overnight expert" following a sighting. But, of course, their "expertise" may well be limited to that specific region, and that particular creature, or group of creatures.
There doesnt need to be a body on a slab, or live specimen in captivity in order for an individual to have some or even extensive. knowledge of these creatures, through sighting events, habituation contexts, interspecies interactions, and communications with others who have also experienced such phenomena.
Should we choose to wait until these beings are officially recognized and acknowledged to further the inquiry, we will find ourselves woefully behind those who moved foreward with the certainty granted by their contact with these creatures that as of yet dont exist...