Changing Equihash parameters from (200,9) to (144,5) would actually be returning to the intended parameters at launch.
Prior to launch, the built-in CPU solver was not optimized, so there was a fear that solutions with (144,5) would take too long to find, given the block interval of 2.5 mins. Also, we didn’t know how much of a speedup GPU miners offered.
Changing the PoW parameters can also have benefits such as improving verification time, which would help Ethereum Solidity contracts to verify Zcash blocks, as well as tphones and constrained devices such as Raspberry Pi’s.
If someone discovered a security bug in the PoW, we would also have to change it.