How sure are we that the hard fork won't cause any catastophic problems, i.e. has it been extensively tested on a testnet and came out rosy?
the hard fork was tested extensively before 0.9 was released, i'm pretty sure.
Along with isolated tests, the testnet was hard forked before 0.9 and has been running the new version ever since.
There are always risks, fork or no fork. Having a fork probably increases risks by some indeterminate amount. If problems happen we'll fix them.