A 622mm rim (aka 700c) is commonly used on road bikes and their kin, most hybrids, and 29" mtbikes. Your hybrid won't be so good with 26" wheels/tires, due to (presumably) issues with brake reach,and other issues related to geometry. For instance, the BB will be considerably lower, and that would be bad in most cases. Worst case would be off-road.

You probably have a lot of clearance for tires, but perhaps not enough to run 29er mountain tires. Measure between the fork and stays, both laterally and vertically. If I were you, and i truly couldn't just buy a nice used mtb for about $100 on CL(And sink another $50 or $100 to fix/tune/replace tires), I'd get some 35mm or slightly wider cyclocross tires. Anything too gnarly for those tires will likely be too gnarly for other components on your bike.

just my $.02
-rob