Hi
The "bulk"(would usually refer to oem, but not in this case) series f/w is intended for third parties using the 1640 as generic (preinstalled in systems) Hi-Val is the only one that I know of that uses this f/w. Upon flashing the warranty is void (this applies to all f/w flashing, as there is a element of the flash not working properly), this is assuming that dealer/manufacturer can tell this has taken place. So yes you can flash back, should you want to at some point.
Personally I've flashed hundreds of drives & so far never had a problem.